Composite Materials

ABSTRACT

A composite material having a plasma frequency comprising a random mixture of conductive and non-conductive particles. A material having smaller conductive than non-conductive particles and a concentration of conductive particles approximately at, close to or above the percolation threshold for mixtures of the conducting and non-conducting particles may show a plasma frequency well below plasma frequencies for conventional bulk materials.

The present invention is concerned with composite materials. Inparticular, preferred embodiments are concerned with metal/insulatorcomposites having plasma frequencies below the plasma frequencies ofconventional bulk metals.

Many applications, devices and/or methods rely on the control ofelectromagnetic radiation. For example, enclosures (radomes) arenecessary to provide environmental protection for antenna systems. Inmobile communications and other similar applications, there is a need toseparate electromagnetic signals of different frequency. There is also aneed to dissipate electromagnetic energy at the walls of anechoicchambers used in radio and microwave measurements, and to confine,within specific bounds, unintentionally emitted electromagnetic energyto meet electromagnetic compliance regulations and preventelectromagnetic interference between electrical and electronicequipment.

Materials are used to provide the means of control, either in bulk form,as coatings or as components in devices. For example, radomes tend to befabricated from bulk materials such as plastics and fibre-reinforcedpolymer composites; frequency separation can be achieved at a componentlevel in guided wave communications or by using coatings (for example onradomes) for free-field propagation; dissipation tends to be achieved bycoating an existing structure (e.g. the walls and floor of an anechoicchamber); and electromagnetic shielding can be achieved either throughcoating an equipment enclosure or by fabricating the enclosure from anappropriate material.

At the simplest level, the role of the material can be to modify thepropagation characteristics of incident radiation. Modification couldinclude transmitting, filtering, absorbing or reflecting incidentelectromagnetic radiation as in radomes, frequency separation, coatingsfor anechoic chambers and equipment enclosures for electromagneticcompatibility.

Advances have also led to materials and devices that amplify or changethe frequency or polarisation of incident electromagnetic radiation(consider, for example, lasers, second harmonic generation usingnon-linear optical materials, or the use of Faraday rotation inferromagnetic ceramics).

Materials and devices also exist whose influence on incidentelectromagnetic radiation can be changed as a function of an extrinsic(or external) stimulus. These are known as smart, dynamic or adaptiveelectromagnetic materials and include ferroelectrics, whose permittivityis a function of applied electric field strength, and chromogenicmaterials (photo-, thermo-, or electro-chromic) whose optical colour andoften electrical conductivity varies with light intensity, temperatureor electrical current.

The thesis, “Electrical Percolation and the Design of FunctionalElectromagnetic Materials” by Ian J, Youngs, published in December 2001,and available from the library of the University of London includes acomprehensive discussion of the background to and physics surroundingthis invention.

The influence exerted by a material on an electromagnetic wave isdetermined by two intrinsic material properties. These are thepermittivity (∈) and magnetic permeability (μ). The permittivity (∈)characterises the response of a material to an applied electric field,and is a measure of the extent to which a material can resist the flowof charge in an electric field. The magnetic permeability (μ)characterises the response of a material to a magnetic field, and isequal to the ratio of the magnetic flux density to the magnetic fieldstrength measured in the material.

It is usual to relate (or normalise) the absolute properties ofpermittivity and magnetic permeability to those of a vacuum (∈ _(o)=8.854×10⁻¹² Fm⁻¹; μ _(o) =1.257×10⁻⁶ Hm⁻¹) so that one then discussesthe relative permittivity (∈ _(r) =∈/∈ ⁰ ) and relative permeability (μ_(r) =μ/μ ⁰ ) of a material. For example, the relative permittivity andrelative permeability of a vacuum equal unity.

The present invention is primarily concerned with responses to anapplied electric field (i.e. permittivity) and the manner in which theygovern the propagation of an electromagnetic wave through the bulk of amaterial. For the purposes of a general introduction, only the behaviourof non-magnetic materials is considered below. This is a reasonableassumption to make, since materials exhibiting diamagnetic orparamagnetic behaviour have a relative magnetic susceptibility (χ _(m) ,a ratio of the magnetic moment per unit volume of material to themagnetic field strength) of |χ _(m) |<10⁻⁵ and so are treated as havinga value of μ _(r) of 1.

In the case of ferromagnetic and ferromagnetic materials, where |χ _(m)| is significantly greater then 0, the analogous case to that outlinedbelow will be apparent to those skilled in the art.

Materials can either support (or allow) the propagation of anelectromagnetic wave through their bulk or they cannot. All materialscontain electronic charges and so respond, to varying degrees, to theapplication of an electric field.

Metals contain significant numbers of electronic charges that are freeto move through the bulk of the material (the conduction bandelectrons). An electric field applied to a metal therefore induces amacroscopic transport current in the material.

The frequency response of the permittivity of metals is determined bythe weakly-bound (“free”) electrons in the conduction band. At lowfrequencies, the electrons oscillate in phase with an applied electricfield. However, at a certain characteristic frequency, oscillation inphase with the applied field can no longer be supported, and resonanceoccurs.

The weakly bound electrons within the metal can be considered to act asa plasma—a gas consisting either wholly or partly of charged particles.A simple example is to consider such an electron gas as being in twodimensions and held between two opposing electrodes, one at the top ofthe plasma and one at the bottom. When an electric field is applied tothis plasma, the electrons will receive enough momentum to move in theopposite direction to that in which the field is applied, and willcontinue to move after the field is turned off.

After time t, N electrons of charge e will have moved a distance, x,producing a sheet of unbalanced charge −Nex at the top of the plasma.Consequently, a region of opposite charge, Nex is left at the bottom ofthe plasma. This results in an electric field, E in the upwarddirection, of magnitude E=(Ne/∈ ⁰ )x acting within the plasma. Thisproduces a restoring force on the electrons, creating an equation ofmotion

$\begin{matrix}{{\frac{^{2}x}{t^{2}} + \frac{N\; ^{2}}{m_{e}ɛ_{0}}} = 0} & (1)\end{matrix}$

where m_(e) is the mass of our electron.

The electrons therefore vibrate at the plasma frequency, ω_(p), where

ω_(p) ²=(e ² /m _(e)∈ ⁰ )N  (2)

For metals, this characteristic frequency is in the ultraviolet regionof the electromagnetic spectrum. For frequencies above ω_(p), metals canbe considered to act like dielectrics, i.e. they have a positivepermittivity and support a propagating electromagnetic wave.

The oscillation of a plasma may be quantised: a plasmon is the unit ofquantisation. Plasmons have a profound impact on the properties of themetal, especially on the effect of incident electromagnetic waves. Theaction of the plasmons produces a complex dielectric function (orpermittivity) of the form

$\begin{matrix}{{ɛ(\omega)} = {1 - \frac{\omega_{p}^{2}}{\omega \left( {\omega + {\gamma}} \right)}}} & (3)\end{matrix}$

The imaginary component arises through the damping term γ, whichrepresents the amount of plasmon energy dissipated into the system,generally as heat. The real permittivity is essentially negative belowthe plasma frequency, ω_(p), at least down to frequencies of the orderof γ.

For frequencies below ω_(p), metals therefore exhibit a negativepermittivity. In this case, an electromagnetic wave cannot propagatethrough the material, and decays exponentially within a characteristicdistance determined by the attenuation coefficient, α=2ωn_(i)/c. In asense, the metal acts as a high-pass filter for the frequency rangespanning the plasma frequency.

For metals, where the frequency of the electromagnetic radiation isbelow the ultraviolet end of the spectrum most of the radiation isreflected and the remainder is attenuated by the metal.

Dielectrics are classed as non-magnetic materials, and contain chargeswhich are mostly bound and whose motion is therefore localised todistances much smaller than the wavelength of the incidentelectromagnetic radiation. The relative permittivity of a dielectricmaterial will be positive and greater than that of a vacuum.

Bound electric charges can exist on many scales within a material, fromelectrons orbiting atomic nuclei to charges residing at interfacesbetween phases of dissimilar chemical composition within a material. Atlow frequencies, all charges will oscillate in phase with an appliedelectric field. This contributes to the maximum value of thepermittivity exhibited by the material. This is shown in a dynamicpermittivity resulting from an applied AC field, rather than thedielectric constant which is representative of an applied DC (or static)field. Under these conditions and in the absence of free electriccharges, the material exhibits no significant loss. Again, at certaincharacteristic frequencies, the individual types of charge carriers nolonger oscillate in phase with the applied field. Maxima in the loss (orabsorption) spectrum occur at these frequencies.

When an E-field is applied to a dielectric material, polarisation of thecharges within the material occurs. The force exerted on an electron bythe electric field, E(t) of a harmonic wave of frequency ω, gives anequation of motion for an electron of

$\begin{matrix}{{{m_{e}\frac{^{2}x}{t^{2}}} + {m_{e}\gamma \frac{x}{t}} + {m_{e}\omega_{0}^{2}x} - {e\; E_{0}\cos \; \omega \; t}} = 0} & (4)\end{matrix}$

where m_(e) is the electron mass, E₀ is the magnitude of the appliedelectric field, ω₀ is the characteristic (or resonance) frequency, ω isthe frequency of the applied electric field, e is the electronic chargeand x is the distance moved by an electron under the influence of theapplied electric field. m_(e)γdx/dt is a damping term representing thedelay between the application of the external field and the time afterwhich an equilibrium in the polarisation is established. Thepolarisation of the material in this field is caused by N contributingelectrons and is given by P=exN, which is related to the permittivity ofthe material, ∈ by

∈=∈₀ +P(t)/E(t)  (5)

Hence the permittivity of the material is given by

$\begin{matrix}{ɛ = {ɛ_{0} + {\frac{^{2}N}{m_{e}}\frac{1}{\left( {\omega_{0}^{2} - \omega^{2} - {\; \gamma \; \omega}} \right)}}}} & (6)\end{matrix}$

Furthermore, there is a relationship between the real (∈′) and imaginary(∈″) parts of the permittivity of a material, given by theKramers-Kronig relations:

$\begin{matrix}{{ɛ^{\prime}(\omega)} = {ɛ_{0} + {\frac{2}{\pi}{\int_{0}^{\infty}\frac{\omega^{\prime}{ɛ^{''}\left( \omega^{\prime} \right)}{\omega^{\prime}}}{\left( {\omega^{\prime 2} - \omega^{2}} \right)}}}}} & \left( {7\; a} \right) \\{{ɛ^{''}(\omega)} = {{- \frac{2\; \omega}{\pi}}{\int_{0}^{\infty}\frac{\left\lbrack {{ɛ^{\prime}\left( \omega^{\prime} \right)} - ɛ_{0}} \right\rbrack {\omega^{\prime}}}{\omega^{\prime 2} - \omega^{2}}}}} & \left( {7\; b} \right)\end{matrix}$

These characteristic frequencies (ω₀) are found experimentally by amaximum in the imaginary permittivity component and represent a regionof absorption over which incident electromagnetic energy is converted toheat through electron-phonon interactions within the material. A phononis an elastic wave caused by harmonic vibrations within the crystallattice.

Over the frequency range containing the absorption band, the realpermittivity component will also be frequency dependent—through theKramers-Kronig relationships. The nature of this frequency dependence isrelated to the level of damping. At high frequencies, generally wellabove the microwave region, the damping effects are greatly reduced andthe polarisation mechanisms are related to the creation of dipoles atelectronic and atomic scales. In this case the real polarisabilitycomponent is of a resonant nature centred on the characteristicfrequency as shown in FIG. 1. At lower frequencies, including themicrowave region, the damping effects are larger and the polarisationmechanisms are related to molecular through to macroscopic scales. Theresponse about the characteristic frequencies tends to that of acritically damped system and the real polarisability component decaysmonotonically with increasing frequency, as shown in FIG. 2. This isknown as dielectric relaxation. These effects can be used to absorb theenergy of incident electromagnetic radiation in a frequency rangecentred on the characteristic frequency, rather than transmitting it orreflecting it back to its source.

The ratio of the imaginary to real components represents the phase lagof the electric component of an incident electromagnetic wave inside thematerial, compared to the electric field component of the incidentelectromagnetic wave outside the material.

At an interface, such as that shown in FIG. 3, it is the relativepermittivity that determines the proportion of incident radiation thatis reflected, shown as r, and the proportion which is transmitted, shownas t. This is given by the Fresnel equations, (where ζ and * indicatecomponents when the incident electric field is perpendicular andparallel to the plane of incidence respectively)

r _(ζ) =[Z ₂ cos(θ_(i))−Z ₁ cos(θ_(t))]/[(Z ₂ cos(θ_(i))+Z ₁cos(θ_(t))]  (8a)

r _(*) =[Z ₁ cos(θ_(i))−Z ₂ cos(θ_(t))]/[(Z ₁ cos(θ_(i))+Z ₂cos(θ_(t))]  (8b)

t _(ζ)=2Z ₂ cos(θ_(i))/[Z ₂ cos(θ_(i))+Z ₁ cos(θ_(t))]  (8c)

t _(*)=2Z ₂ cos(θ_(i))/[Z ₁ cos(θ_(i))+Z ₂ cos(θ_(t))]  (8d)

where Z₂=μ_(r)/∈_(r) and the subscripts 1 and 2 refer to the materialseither side of the interface, with material 1 containing the incidentelectromagnetic wave. Material 1 is often air in which caseZ₁=μ_(r1)=∈_(r1)=1. If material 2 is non-magnetic then μ_(r2)=1, also.

For example, for metals in air, most of the incident radiation in themicrowave and visible regions of the spectrum (frequencies in the regionof approximately 10⁸ to 10¹⁵ Hz) is reflected. For example, thereflectivity of freshly deposited aluminium, in air, is around 94% to99% for wavelengths between 10 and 30 μm.

The angles of incidence (θ_(i)) and refraction (θ_(t)) in theseequations are given by Snell's law,

n₁ sin θ_(i)=n₂ sin θ_(t)  (9)

where n₂=∈_(r)μ_(r) and the subscripts 1 and 2 are as defined for Z inthe Fresnel equations.

It is clear then that identifying materials with differentpermittivities can enable the design of components and devices withdifferent electromagnetic functionality (for example, different levelsof reflection, transmission and absorption) operating over specificregions of the electromagnetic spectrum. However, the range of naturallyoccurring permittivities has become restrictive to the design engineer.For example, either because the desired real permittivity value is notavailable or absorption mechanisms do not exist at a required frequency,or in a material that has the required processibility, mechanical,environmental or visual properties. For these reasons, engineers havesought to form composite media with tailored complex permittivity. Forexample, and for many years, high permittivity materials and metals havebeen added, in powdered forms, to polymers and other low permittivityhost materials (matrices) (e.g. ceramics and glasses) to raise the basepermittivity of the host or to engineer absorption (e.g. throughelectrical resistance or the Maxwell-Wagner-Sillars effect). Thepermittivity of these composite media is now considered an ‘effective’permittivity. For this to be valid, and the composite medium to betreated as a homogeneous material for design purposes, the size of theinclusions must be smaller (and ideally) much smaller than thewavelength of interest.

Work has also been done on trying to design solid materials that have aplasma frequency at lower frequencies than naturally occur in metals.

It has been known for some time [Bracewell R, Wireless Engineer, p. 320,1954], that periodic arrays of metal elements can be used to formcomposite media with low plasma frequencies. More recently [Pendry J etal, Physical Review Letters, vol. 76, p. 4773, 1996] it was demonstratedthat a periodic lattice of thin metallic wires could exhibit a plasmafrequency given by;

ω_(p)≈2πc ²/(d ² ln(d/r))  (10)

in the microwave region when the wire radius (r) is much smaller thanthe wire spacing (d), and c is the speed of light in vacuum. Forexample, when the wire radius is 20 μm and the wire spacing is 5 mm, theplasma frequency is approximately 10 GHz.

There have been no other experimental observations of plasma resonancesat microwave frequencies in naturally occurring materials or artificialcomposites other than in the fine wire system discussed above. However,there is evidence of low frequency plasmons in the infrared region ofthe electromagnetic spectrum. For example, low frequency plasmons havebeen observed in intrinsically conducting polymers (Kohlman R et al,Chapter 3, Handbook of Conducting Polymers, Second Ed., Ed. Skotheim,Elsenbaumer and Reynolds, Marcel Dekker, New York 1998 ISBN0-8247-0050-3), in coupled metallic island structures (Govorov et al,Physics of the Solid State, vol. 40, p 499, 1998) and in metallicphotonic bandgap crystals (Zakhidov et al. Synthetic Metals, vol. 116, p419, 2001).

It is also known how to create artificial dielectric structures, forexample, for use as radar antennas (see Skolnik, Introduction to RadarSystems, McGraw-Hill, London, 1981, Martindale, J. Brit. IRE, vol. 13, p243, 1953, Stuetzer, Proc. IRE, 38, p 1053, 1950, and Harvey, Proc. IRE,vol. 106 Part B, p 141, 1959). An artificial dielectric comprisesdiscrete metallic particles of a macroscopic size. For example, theseparticles may be spheres, disks, strips or rods embedded within amaterial of low dielectric constant, such as polystyrene foam. Theseparticles are arranged in a three dimensional lattice configuration,with the dimensions of the particles in the direction parallel to theapplied electric field, as well as the spacing between the particles,being of an order comparable with the incident wavelength. For a smallconcentration of metallic spheres of radius r and spacing d, andassuming there is no interaction between the spheres, the dielectricconstant of an artificial dielectric is approximately

κ=1+4πr ³ /d ³  (11)

(The symbol κ has been used here to represent the dielectric constant toavoid confusion with the use of the symbol ∈ to represent permittivity.)

An artificial dielectric may also be constructed using a soliddielectric material that comprises a controlled arrangement of sphericalor cylindrical voids.

Leaving behind the assumption of non-magnetic materials taken above, andfollowing on from wire arrays, it is also possible to producealternative periodic arrangements of metallic elements which exhibitnegative magnetic permeability also at microwave frequencies (Pendry Jet al., IEEE Transactions on Microwave Theory and Techniques, vol. 47, p2075, 1999). A combination of these two techniques has also led to realmaterials exhibiting “left-handed” electromagnetic behaviour or anegative angle of refraction (Smith D R et al., Phys. Rev. Lett., vol.84(18), p 4184, 2000.).

Very recently a theoretical model has led to speculation [Holloway etal, IEEE Transactions on Antennas and Propagation, Vol 51, No. 10,October 2003] that it may be possible to produce double negative media(i.e. having effective permeability and permittivity simultaneouslynegative) by producing a composite material consisting of insulatingmagnetodielectric spherical particles embedded in an insulatingbackground matrix.

The advantages of producing a material with a negative magneticpermeability are similar to those found on producing a negativepermittivity. So far, we have only considered the electric component ofan applied electromagnetic field, but any material which produces a losswhen exposed to an applied electromagnetic field can do so via theelectric or the magnetic component of that field, or both. The bestmaterials which exhibit losses via the magnetic field component areferrites. These materials show ferromagnetism, where saturationmagnetisation does not correspond to parallel alignment of the magneticmoments within the material. Such materials also tend to have a spinelcrystal structure, comprising 8 occupied tetrahedral sites and 16occupied octahedral sites within a unit cell. For example, magnetite,Fe₃O₄ or FeO.Fe₂O₃ comprises both ferric (Fe³⁺) and ferrous (Fe²⁺) ions.At saturation, the moments of all of the Fe³⁺ ions on the tetrahedralsites and of the Fe³⁺ ions filling 8 of the octahedral sites are alignedantiparallel, thus cancelling each other out. The residual magneticmoment is therefore only contributed to by the Fe²⁺ ions on theremaining octahedral sites. Such a material has a complex permeability,

μ=μ′+iμ″  (12)

where μ′ is the real component and μ″ the imaginary component.

Consequently, it is also desirable to find materials with a negativepermeability, since in these the magnetic component of theelectromagnetic wave will die away exponentially within the material.

Although a single period of a fine wire array of the type proposed byPendry J. et al in Physical review letters, 76, 9773, 1996 is smallerthan the incident wavelength (0.03 m at 10 GHz, with λ/d≈6) it is notmuch smaller (an order of magnitude) than the wavelength. In practice,more than one period of such a structure may be required in thedirection of propagation of an electromagnetic wave for the effectivepermittivity of such a composite medium to be a valid representation ofthe electromagnetic response of that medium. Consequently, this couldnot be considered to be “thin” in comparison with the wavelength at theplasma frequency. This may be a limiting factor to the use of such mediain practical applications. Media of the type proposed by Pendry J. et alwould also be difficult and expensive to produce.

A further benefit to the design engineer would be realised if it werepossible to produce composite media with a tailored plasma frequency.Particularly, if in a solid material, the plasma frequency could betailored to exist at lower frequencies than naturally occur in metals.For example, work has recently been reported in the scientificliterature where this has been achieved in the microwave or even radiofrequency regions of the electromagnetic spectrum.

There is, therefore, a need to develop alternative composite media whichexhibit metallic-like permittivity spectra with a plasma frequency wellbelow that of conventional bulk metals, which do not depend on the useof components and spacings of the components with dimensions related tothe wavelength of interest, whose effective permittivity is realisableon a scale much smaller than the wavelength of interest, and which maybe more easily manufactured than the wire structures discussed above.

The present invention, in its various aspects, provides a compositematerial, use of a composite material product, device or apparatus, or amethod as defined in one or more of the attached independent claims towhich reference should now be made.

Further preferred features of the invention are set out in the dependentclaims to which reference should also be made.

The invention in a first aspect provides a composite material accordingto claim 1.

It is known to make composites comprising mixtures of electricallyconductive and non-electrically conductive particles (see, for example,EP 779,629 or U.S. Pat. No. 4,997,708). However, such known compositeswould not exhibit a plasma frequency. The known composites of thisnature are mostly reflective to incident radiation below opticalwavelengths. Composites embodying the present invention could bereflective, absorbing or exhibit filtering characteristics similar toelectromagnetic bandgap structures.

In the claims and description, the term random is intended to meanwithout order. The electrically conductive material need not beuniformly dispersed and there could be portions of the material in whichthere is localized order of the electrically conductive material.

In a preferred embodiment, the electrically conductive material has nolong range order within the composite material. By long range order, itis intended that there is no regularity of structure (crystal orotherwise) for the electrically conductive material. Consequently thereis no regularity of crystal structure, or periodic lattice structurepresent of the conductive material within the composite material.

As discussed in more detail below an alternative definition of what ismeant by no long range order is no order at or above the dimensionscorresponding to the effective wavelength of electromagnetic radiationpropagating in the material.

The invention in another aspect provides a composite material comprisingan electrically conductive material and a non-electrically conductingmaterial, wherein the concentration of electrically conductive materialis approximately at, close to or above its percolation threshold.

A discussion of how to achieve percolation threshold is set out in apaper by the inventor (Ian J. Youngs) “A geometric percolation model fornon-spherical excluded volumes”—Journal of Physics D: Applied Physics 36(2003) p. 738-747.

The inventor has appreciated that the existing theoretical models of thebehaviour of composite material comprising mixtures of conductive andnon-conductive or insulating materials are wrong. The inventor is thefirst to establish that such materials may have a plasma frequency belowthat of conventional bulk materials.

Preferably the composite material comprises particles of electricallyconductive and non-electrically conductive materials. Such materials areeasy to make.

Preferably, the particles are randomly distributed. The inventor is thefirst to appreciate that composite materials need not have a regularstructure of the type previously thought necessary (see, for example,Physical Review Letters, vol. 76, p. 4773, 1996] Pendry et al) tocontrol or alter the plasma frequency.

Preferably, the particles are small, with the conductive particles beingsmaller than the non-electrically conductive particles. The reasons forthe behaviour of the composites of the investigation are as yet notfully understood and investigations are ongoing. However, it appearsthat composites in which spaces of insulating material (e.g. anon-conductive particle or area) are surrounded by conductive particles(e.g. a coating of conductive particles on an insulating ornon-conductive particle) are particularly advantageous.

Preferably the conductive particles are resistant to oxidation andpassivation. Small particles are more reactive than larger particles andit is therefore advantageous to have particles whose surface will notreact so as to try and ensure that the conductive particles' behaviour(e.g. conductivity) is not altered or affected by surface effects suchas oxidation.

Preferably the oxidation resistant particles are noble metals,conducting ceramics or metallic alloys.

Preferred embodiments of the present invention will be described, by wayof example only, with reference to the attached figures. The figures areonly for the purposes of illustrating one or more preferred embodimentsof the invention and are not to be construed as unifying the inventionor limiting the invention or limiting the appendent claims. The skilledman will readily and easily envisage alternative embodiments of theinvention in its various aspects.

In the figures:

FIG. 1 illustrates the high-frequency permittivity of a typicaldielectric material centred on a resonance frequency;

FIG. 2 illustrates the low-frequency permittivity component of a typicaldielectric material centred on a relaxation frequency;

FIG. 3 shows an interface between two media for illustrating the Fresnelequations;

FIG. 4 shows the theoretical electromagnetic properties of a compositematerial with a filler conductivity of 1×10⁷ S/m in a matrix with apermittivity of 2.1−j0.001 at 1 GHz predicted using Maxwell-Garnettmixture law;

FIG. 5 shows the theoretical electromagnetic properties for thecomposite material of FIG. 4, but with a filler volume fraction orconcentration of 99.9 vol % predicted using Maxwell-Garnett mixture law;

FIG. 6 shows the theoretical variation of composite permittivity andconductivity with filler volume fraction or concentration predictedusing the Bruggeman model;

FIG. 7 illustrates the theoretical variation in permittivity andconductivity under percolation theory using the Bruggeman model;

FIG. 8 shows the theoretical variation in permittivity and conductivityfor a composite with a filler volume fraction or concentration of 33.3vol %, using the Bruggeman model;

FIGS. 9 a to 9 f illustrate, respectively, the theoretical variations inthe real permittivity, imaginary permittivity, conductivity, dielectricloss tangent, real electric modulus and imaginary electric modulus forcomposites with filler concentrations above and below the percolationthreshold predicted using the Bruggeman model;

FIGS. 10 a-10 d illustrate, respectively, the experimentally determinedvariation in the real permittivity, conductivity, dielectric losstangent and imaginary electric modulus respectively of nano-aluminium inPTFE composites;

FIGS. 11 a-11 d illustrate, respectively, the experimentally determinedvariation in the real permittivity, conductivity, dielectric losstangent and imaginary electric modulus respectively of nano-silver in100 μm PTFE composites

FIGS. 12 a-12 d illustrate, respectively, the experimentally determinedvariation in the real permittivity, conductivity, dielectric losstangent and imaginary electric modulus respectively of nano-silver in 1μm PTFE composites

FIGS. 13 a to 13 d illustrate the experimentally determined variation ofthe real permittivities in the microwave region for differentnano-silver in 100 μm PTFE composites;

FIGS. 13 e, 13 f and 13 g illustrate the experimentally determinedvariation in conductivity, real permeability and imaginary permeability,respectively, for different nano-silver in 100:m PTFE;

FIGS. 14 a to 14 d illustrate the experimentally determined variation ofpermittivity in the microwave region for different nano-silver in 1 μmPTFE composites;

FIGS. 14 e, 14 f and 14 g illustrate the experimentally determinedvariation in conductivity, real permeability and imaginary permeability,respectively, for different nano-silver in 1:m PTFE composites;

FIG. 14 h is a comparison of the filler concentration dependence ofconductivity for different silver-filled composites, highlightingvariations in the gradient of the percolation (insulator-conductor)transition (solid data points and data points with a backgroundrepresent samples exhibiting a plasma-like response).

FIGS. 15 a and 15 b illustrate the experimental complex permittivityspectrum of a titanium diboride PTFE composite;

FIGS. 16 a to 16 d illustrate the experimentally determined dielectricresponse of nano-copper PTFE composites;

FIGS. 17 a to 17 d illustrate the experimentally determined dielectricresponse of nano-cobalt PTFE composites;

FIGS. 18 a to 18 d illustrate the experimentally determined microwavemagnetic permeability spectra of cobalt PTFE and cobalt wax composites;

FIGS. 19 a to 19 d illustrate the fit between experimental data andmodelled or theoretical data using the fitting parameters given in Table3;

FIGS. 20 a to 20 d illustrate the fit between experimental data andmodelled or theoretical data using the fitting parameters given in Table4;

FIGS. 21 a to 21 h show SEM (scanning electron microscope) images ofPTFE and nano-silver particles composites;

FIGS. 22 a to 22 d show a further fit between experimental data andmodelling or theoretical data using the fitting parameters given inTable 5;

FIG. 23 a shows low frequency conductivity measurements for nano-silvercompositions; and

FIG. 23 b shows low frequency real permittivity measurements for thenano-silver samples of FIG. 23 a.

FIG. 24 is a schematic graph of the insulator-to-metal transition forcompositions with matrix particle sizes of 1 μm and 100 μm;

FIG. 25 is a graph comparing the concentration dependence of theconductivity of four silver-based compositions at 0.5 GHz;

FIG. 26 is a graph showing scaling of the real permittivity of a sampleof 100 nm Ag/100 μm PTFE composite;

FIG. 27 is a graph showing scaling of the conductivity of a sample of100 nm Ag/100 μm PTFE composite;

FIG. 28 is a graph showing scaling of the real permittivity of a sampleof 100 nm Ag/1 μm PTFE composite;

FIG. 29 is a graph showing scaling of the conductivity of a sample of100 nm Ag/1 μm PTFE composite;

FIG. 30 is a graph of frequency dependent conductivity of a 2 vol % 100nm Ag/100 μm PTFE composite over the range 1 Hz to 1 MHz and power lawanalysis;

FIG. 31 is a graph of frequency dependent real permittivity of a 2 vol %100 nm Ag/100 μm PTFE composite over the range 1 Hz to 1 MHz and powerlaw analysis;

FIG. 32 is a graph of frequency dependent conductivity of a 8 vol % 100nm Ag/1 μm PTFE composite over the range 1 Hz to 1 MHz and power lawanalysis

FIG. 33 is a graph of frequency dependent real permittivity of a 8 vol %100 nm Ag/1 μm PTFE composite over the range 1 Hz to 1 MHz and power lawanalysis;

FIG. 34 illustrates dielectric response;

FIG. 35 is a summary of experimental results in terms of measuredconductivity at 0.5 GHz;

FIG. 36 is a graph showing the temperature dependence of theconductivity of samples of 1 vol % 100 nm Ag/100 μm PTFE composite (2samples);

FIG. 37 is a graph showing the temperature dependence of theconductivity of samples of 2 vol % 100 nm Ag/100 μm PTFE composite (3samples);

FIG. 38 is a graph showing the temperature dependence of theconductivity of samples of 3 vol % 100 nm Ag/100 μm PTFE composite (3samples);

FIG. 39 is a graph showing the temperature dependence of theconductivity of samples of 5 vol % 100 nm Ag/100 μm PTFE composite (2samples);

FIG. 40 is a graph showing the temperature dependence of theconductivity of samples of 2 vol % 100 nm Ag/1 μm PTFE composite (1sample);

FIG. 41 is a graph showing the temperature dependence of theconductivity of samples of 8 vol % 100 nm Ag/1 μm PTFE composite (2sample);

FIG. 42 is a graph showing the temperature dependence of theconductivity of samples of 10 vol % 100 nm Ag/1 μm PTFE composite (2samples);

FIG. 43 shows graphs of ln(conductivity)v 1/T and ln(conductivity) vln(temperature) for the samples of FIG. 37;

FIG. 44 shows graphs of ln(conductivity)v 1/T and ln(conductivity) vln(temperature) for the samples of FIG. 38; and

FIG. 45 shows graphs of ln(conductivity)v 1/T and ln(conductivity) vln(temperature) for the samples of FIG. 42.

FIG. 46 illustrates the percolation threshold for a composite material,

FIGS. 47 a to 47 c illustrate three different conductive patterns madeup of circular conductive elements for placing on a dielectricsubstrate.

FIG. 48 illustrates an alternative conductive pattern made up of crosseddipoles or crosses; and

FIGS. 49 and 50 illustrate two possible methods of making atwo-dimensional composite material using conductive patterns of the typeshown in FIGS. 47 and 48.

The inventor of the subject invention is the first to appreciate, afterextensive research and investigation, that it is possible to produce amaterial having a plasma frequency below the plasma frequencies ofconventional bulk metals. The inventor is the first to establish thatmaterials comprising electrically conductive particles within aninsulating host medium can have a plasma frequency below that ofconventional bulk metals.

Although it is known (See Kiesow et al, Journal of Applied Physics, Vol.94, number 10-15 Nov. 2003) that plasma polymer films with embeddedsilver nanoparticles can exhibit a reversible electronic switchingeffect, the inventor of the subject application is the first to realisethat it is possible to create materials having a plasma frequency belowthat for conventional bulk metals using composite metals comprising amixture of electrically conductive and electrically non-conductiveparticles in the manner set out in the claims of the subjectapplication.

Some embodiments of the present invention are developed from anon-periodic and generally random distribution of conducting particleswithin an insulating host medium. The conducting particles may be ametal, metal alloy, conductive metal oxide, intrinsically conductivepolymer, ionic conductive material, conductive ceramic material or amixture of any of these. In preferred embodiments the conductingparticles are stable against oxidation and passivation and are, forexample, noble metals such as silver or gold, metallic alloys orconducting ceramics (titanium diboride).

The insulating material may be particles of polytetrafluoroethylene(PTFE), paraffin wax, a thermosetting material, a thermoplasticmaterial, a polymer, an insulating ceramic material, glass or a mixtureof insulating materials. The insulating material could also be air, orcontain trapped air.

Investigations into the performance of different composites are ongoing.Presently, the inventor has determined that composite materialscomprising a mixture at approximately its percolation threshold ofconductive particles in the size range 1 nm to 1 μm and largernon-conductive particles (preferably at least 10 times as large asconductive particles) have particularly desirable properties. Forexample and as discussed in more detail below, silver particles havingan average size of 100 nm (as determined using specific surface areameasurements (BET)) randomly distributed in a PTFE host made up of PTFEparticles having an average size of 100 μm. (Aldrich 468811-8).

The nano silver in PTFE composite may be made by mixing particles of thetwo constituent elements to form a mix, forming the mix to produce apreform and recovering the composite material.

The composite may be made by the methods described below in connectionwith the experiments carried out by the inventors (see experiments 1 to3). In these methods powders are mixed and then die-pressed at apressure in the range 130-260 MPa for a period in the range 80-300second.

Although in the experiments the powder mixtures were die-pressed at roomtemperature, the temperature used to press the medium may be variedaccording to the polymer used, and should be sufficient to allowpreferable conductive particle coating of non-conductive matrix byinducing mechanically or thermally induced flow. Pressure and time mayalso be varied accordingly. Other methods of consolidating a powderfeedstock include extrusion and flame-spraying.

Alternatively, the conducting powder could be dispersed by stirring intoa carrier material such as a thermoplastic at a temperature above itsmelting point, or after the thermoplastic has been dissolved in asuitable solvent, or paraffin wax. The conducting particles could bemixed with a thermosetting polymer prior to curing (by chemical or othermeans). The conducting particles could be formed in situ within apolymer phase by chemically or electrochemically reducing an appropriateprecursor. The conducting powder could be mixed with insulating ceramicor glass powder, compacted and then sintered to form a consolidatedceramic or glass component.

It is possible that any of these systems could be formed into a foam(blown or syntactic or a hybrid of both), in which case the conductingparticles would reside in the cell walls. The foam may be blown usingair or an inert gas (for example, Argon). In ceramic systems it could bepossible to form the conducting phase during the sintering reactions andfor the conducting phase to reside at grain boundaries within theresulting ceramic. A further possibility is to form a metallic foam inwhich case the insulating phase could be air. Again this could beachieved by blowing or syntactically by the addition of hollow particlesabove the melting point of the metal or a hybrid combination of the twomethods. In addition, it may be beneficial to influence the connectivityof the conducting phase through the application of an external stimulussuch as an electric or magnetic field during the consolidation orsolidification process.

By connectivity, it is intended to mean any form of connection betweenparticles or other constituents which forms an electrical connection. Itis not necessary therefore that the particles or constituents should bein physical contact, but an electrical connection could be made even ifthere was a distance of the order of a few nanometers between theparticles or constituents. This would increase the probability ofelectron tunneling or hopping between particles or constituents,resulting in charge transfer. In particular, any electrical conductivitybetween particles in the form of a network, must extend over a distancegreater than the order of the wavelength corresponding to the plasmafrequency in the material.

Although the preparation of the samples is described in the experimentson a laboratory scale, it would be possible to use various known methodsof materials processing on an industrial scale, including, but notlimited to, injection moulding, extrusion, spraying or casting.

The results of experiments 1 to 3 (see below) show that it is possibleto produce composite materials exhibiting a plasma-like response bydispersing silver nano-particles with micron-sized or larger PTFEparticles, or micron sized titanium diboride particles with larger PTFEparticles. The effect appears to be more reproducible when theconducting particle size is significantly smaller than the insulatingparticle size. This may be because it is easier and more reproducible toform conductive networks around and between larger non-conductiveparticles if the conductive particles forming this network are small incomparison.

A further benefit of using conducting particles that are much smallerthan the insulating particles would appear to be a significant reductionin the critical conducting particle concentration—the percolationthreshold—and more reproducible control of insulator/conductormorphology.

However, particle size per se does not appear to be a first order causeof the observed effects, but it is the nature of the inter-particlecontacts and formation of a percolated microstructure which arecritical, as illustrated by the particle size difference effectsdiscussed above and in connection with the experiments discussed below.However, the ratio of sizes of conductive to non-conductive particlesmay be less than, equal to or greater than unity.

Further materials systems that may be of use are excluded volume systems(which utilise small filler concentrations), conductor coated particlesand impregnated ceramic materials. Foams and other well known insulatingmatrices may also be of use. Other ceramic materials, including thosewhere a second phase (for example a conducting phase) is included atgrain boundaries may also be suitable for use with the invention, forexample, Zinc Oxide (ZnO) thin films. Metal-matrix composites may alsobe of use.

In addition, it is proposed that the combination of the currentinvention with a component that exhibits negative magnetic permeabilityover a frequency range where the permittivity is also negative (i.e.below the plasma frequency) would result in a material with a negativerefractive index over the same frequency range. A suitable magneticmaterial would be a ferromagnetic substance: For example the replacementof the purely conductive filler particles discussed above withferromagnetic metal particles such as cobalt, iron or nickel or theiralloys. Such a material would exhibit a negative permeability ifinherent damping mechanisms were sufficiently suppressed or excluded.

The ferromagnetic material could be added to the insulator phase priorto the formation of the negative permittivity composite as shown by wayof example in Experiments 1 and 2. Alternatively, if the ferromagneticcomponent has sufficient electrical conductivity then it could be usedin place of the silver or titanium diboride to form a composite withsimultaneous negative permittivity and permeability.

The effective properties of composites comprising a random distributionof conductively particles in an insulating host medium may be predictedusing mixture laws (also referred to as effective medium theories), ofwhich there are many (Priou A Dielectric Properties of HeterogeneousMaterials, Elsevier, New York, 1992; Neelakanta P Handbook ofElectromagnetic Materials, CRC Press, New York, 1995; Youngs IElectrical Percolation and the Design of Functional ElectromagneticMaterials, PhD Thesis, University of London, 2001). In the majority ofcases, selection of an appropriate mixture law is achieved empirically.It is possible to relate different mixture laws to specific combinationsof particle shape, orientation and microstructural arrangement. However,it can be difficult to pre-determine the microstructural arrangementthat will result from a particular combination of components because theparticle arrangement will be influenced by surface chemistry andprocessing conditions.

Bearing in mind the above limitations, it is possible to select a smallnumber of mixture laws that enable the engineer to explore thequalitative nature of the filler concentration and frequency dependenceof complex permittivity that can be expected for these composites, evenif the laws may be quantitatively incorrect.

It will become clear in the following analysis that one of the existingmixture laws suggests that metals of the type claimed would result inplasma frequencies lower than that of conventional bulk metals. Theinventor is the first to appreciate the advantageous properties of theclaimed materials. The following discussion of the existing mixture lawsclearly demonstrates how no-one would have considered creating or usingmaterials as claimed in this application.

The earliest mixture laws were developed on the assumption of dilutefiller concentrations, with the separation between filler particlesbeing large compared to their radius. A good example is that due toMaxwell-Garnett (Maxwell-Garnett J. ‘Colours in metal glasses and inmetal films’. Philosophical Transactions of the Royal Society, CCIII,pp. 385, 1904.)

The Maxwell-Garnett model or mixture law defines how the overallpermittivity ∈ of the composite material is related to the permittivityof the filler ∈_(f), the permittivity of the matrix ∈_(m), and thefiller volume fraction V:

$\begin{matrix}{ɛ = {ɛ_{m} + {3\; ɛ_{m}\frac{V\frac{\Delta \; ɛ}{ɛ_{f} + {2\; ɛ_{m}}}}{1 - {V\frac{\Delta \; ɛ}{ɛ_{f} + {2\; ɛ_{m}}}}}}}} & (13)\end{matrix}$

with Δ∈=∈_(f)−∈_(m). If the filler is a metal then its permittivity maybe approximated using the low frequency form of the Drude model

$\begin{matrix}{ɛ_{f} = {1 - {\frac{\sigma_{f}}{2\; \pi \; f\; ɛ_{o}}}}} & (14)\end{matrix}$

Where σ_(f) is the filler conductivity.

The filler volume fraction dependence of the relevant effectiveelectromagnetic properties (real and imaginary components ofpermittivity, and conductivity) for a representative theoreticalcomposite with a filler conductivity (σ_(f)) of 1×10⁷ S/m and a matrixpermittivity of 2.1−j0.001 is illustrated in FIG. 4 (using theMaxwell-Garnett model) for a frequency of 1 GHz.

It is observed that both components of permittivity and conductivityincrease with increasing filler volume fraction from those of the matrixto those of filler. In particular, it is observed that the composite hasproperties close to those of the filler phase when the filler volumefraction or concentration is very close to 100%. Intuitively, this isincorrect for a composite containing mono-disperse filler particles,especially in terms of the composite conductivity, because it is to beexpected that the composite conductivity would approach that of thefiller component as soon as the particles touch—i.e. at close-packing,which occurs for filler concentrations in the range 52 to 74 vol. % forspherical particles. Nevertheless, it is recalled that theMaxwell-Garnett model was developed under the assumption of dilutefiller concentrations.

The frequency dependence of the effective electromagnetic properties forthe same composite at a filler concentration of 99.9 vol. %, derivedusing Maxwell-Garnet theory, is illustrated in FIG. 5. A relaxation-typedielectric response similar to that shown in FIG. 2 is observed. Therelaxation frequency is at approximately 10 THz (10×10¹²—i.e. above themicrowave range, which is approximately 10⁸ to 10¹² Hz).

An important advance was made by Bruggeman (Bruggeman D. “Annalen derPhysik Leipzig”, vol 24, p 636, 1935_(e)). Bruggeman sought to overcomethe dilute approximation by treating the filler particles as beingdispersed within a background medium that had the permittivity of themixture rather than the permittivity of the insulating phase. This ledto the following equation, known as the Bruggeman symmetric mixture lawor effective medium theory.

$\begin{matrix}{{{\left( {1 - V} \right)\frac{ɛ - ɛ_{m}}{{2\; ɛ} + ɛ_{m}}} + {V\frac{ɛ - ɛ_{f}}{{2\; ɛ} + ɛ_{f}}}} = 0} & (15)\end{matrix}$

FIG. 6 illustrates the theoretical filler volume fraction concentrationdependence of the real (∈′, σ_(f)′) and imaginary (∈″,σ_(f)″) componentsof permittivity and conductivity for the same representative composite(i.e. with a filler conductivity σ_(f) of 1×10⁷ S/m, a matrixpermittivity ∈_(m) of 2.1−j0.001 and for a frequency of 1 GHz). Thisfigure may be compared directly to FIG. 4.

The Bruggeman model predicts that the properties of the mixture increasedramatically at a critical filler concentration that is much smallerthan the concentration for close packing. This critical concentration isgenerally referred to as the percolation threshold (Vc). The Bruggemanmodel predicts (see FIG. 6) for spherical particles randomly filling acubic lattice, percolation is predicted to occur at a filler volumefraction of approximately 35%. In fact, for real composite materialscomprising spherical particles randomly filling a cubic lattice,percolation is reached at the much lower volume fraction ofapproximately 16%. The Bruggeman theory is therefore quantitativelywrong insofar as prediction of the critical threshold volume fillerfraction V_(c) is concerned. It is however qualitatively correct in thatthe percolation threshold of the material is important, since itrepresents the filler volume fraction at which the composite system willundergo an insulator-to-conductor transition. It is expected that thecomposite material would exhibit insulator-like properties for fillerconcentrations below the percolation threshold and potentiallymetal-like properties for filler concentrations above it.

Percolation theory is a way of describing the processes, properties andphenomena in a random or disordered system. The amount of disorder isdefined by the degree of connectivity between particles. If p is aparameter that defines the degree of connectivity between variousparticles in a material, then if p=0, none of the particles areconnected, and if p=1, all the particles are connected to the maximumnumber of neighbouring particles. There is a point, p_(c) (thepercolation threshold), where each of the particles is connected to theminimum number of neighbouring particles, such that there is asufficiently long unbroken path of that type of particle for current toflow in the material.

In a metal matrix composite, where, e.g. aluminium particles aredispersed in a ceramic matrix, the percolation threshold for appliedD.C. (Direct Current) is reached when there is at least one continuouspath of aluminium from one side of the matrix to the other. In a similarmetal matrix composite the percolation threshold for applied A.C.(Alternating Current) is reached when there are sufficiently long pathsaround particles at the ends of the matrix, for electrons to move as faras is possible in each direction of cycle of applied current before thedirection of applied current is reversed. In other words, the paths aresufficiently long for electrons to move as far as the phase of theapplied alternating current allows them. At this point, the material maybegin to exhibit metallic characteristics; for example, an electriccurrent may flow.

The behaviour of random materials, for example those showing no form ofordering or periodic structure, such as powder systems, near theirpercolation threshold has been widely studied, both experimentally andtheoretically. It is apparent that, for perfectly random systems, thereare a number of features associated with their behaviour over a narrowconcentration range about the percolation threshold.

Many of these features are related to the power-law response observed insystems exhibiting percolative behaviour and the fact that the exponentsin these power-laws appear independent of the precise nature of thematerial, except for the dimensionality of the connectivity betweenparticles. A macroscopic example of this is the filler volume fractionor concentration dependence of the real permittivity and conductivityfor a conductor-insulator composite near the percolation threshold.Percolation theory suggests the following power-laws:

$\begin{matrix}{{ɛ^{\prime} \propto \left( {V - {Vc}} \right)^{- s}},{{\sigma \propto {\left( {V - {Vc}} \right)^{t}\mspace{14mu} {and}\mspace{14mu} ɛ^{''}}} = \frac{\sigma}{{\omega \cdot ɛ}\; o}}} & (16)\end{matrix}$

Where ∈′ is real permittivity, V is the volume fraction of the filler,Vc is the critical filler volume fraction corresponding to thepercolation threshold and σ is conductivity.

FIG. 7 illustrates this point using the data presented in FIG. 6 andcalculated using the Bruggeman mixture law. The logarithm of eachproperty is plotted against the logarithm of a normalised filler volumefraction (V−Vc)/Vc. The data for real permittivity ∈′ is for fillervolume fractions leading up to the percolation threshold. The data forthe imaginary permittivity ∈″, and conductivity σ are for filler volumefractions above the percolation threshold filler volume fraction Vc. Thegradients in FIG. 7 provide the values for the exponents set out inequation (16) above. It is deduced that the Bruggeman mixture lawpredicts that both s and t equal unity. It is at this point that theBruggeman model deviates from percolation theory on a quantitativelevel. Percolation theory predicts that for particles connected on athree-dimensional network, the exponents should have the followingvalues: s=0.73 and t=1.9.

FIG. 8 illustrates the frequency dependence of the effectiveelectromagnetic properties for the same composite at a filler volumefraction V of 33.3 vol. %, calculated using the Bruggeman mixture law.In terms of the normalised filler concentration (V-Vc)/Vc definedpreviously, this concentration is equivalent to that presented in FIG. 5for the Maxwell-Garnett mixture law. It is observed that the Bruggemanmixture law predicts a much broader and non-Debye relaxation peak forthis filler concentration which is close to but below the percolationthreshold. This peak may be characterised by two characteristicfrequencies ω_(ξ) and ω_(MWS) that mark the clear changes in gradientvisible in all three parameters shown in FIG. 8. In addition, since thedata is already plotted on log-log scales, it is observed that the datacovering the central frequency range, defined by these twocharacteristic frequencies, also obeys a distinct power-law response. Inthis case, the gradients all equal one half.

Percolation theory predicts such a power-law response, with therelationships:

∈′(ω,V=Vc))∝ω^(−γ) and σ(ω,V=Vc))∝ω^(x)  (17)

furthermore, that these exponents x, y are related to the exponents s, t(see above) for the concentration dependence by:

$\begin{matrix}{{{x + y} = 1},{x = {\frac{t}{s + t} \approx 0.72}},{y = {\frac{s}{s + t} \approx 0.28}}} & (18)\end{matrix}$

Again, it is noted that the Bruggeman model is quantitatively incorrect,yet self-consistent.

The loss angle δ (where tan(δ)=∈″/∈′, and ∈″ is the imaginary part ofthe permittivity, and ∈′ is the real part) attains a constant valuegiven by yπ/2 for the frequency range between the two characteristicfrequencies, which may be specified as

$\begin{matrix}{{\left( {V - {Vc}} \right)^{s + t}\frac{\sigma_{f}}{ɛ_{o}ɛ_{m}}} \cong \omega_{\xi} \leq \omega \leq \omega_{MWS} \cong \frac{\sigma_{f}}{ɛ_{o}ɛ_{m}}} & (19)\end{matrix}$

The term (V−V_(c))^(s+t) is a weighting to indicate how close acomposition is the percolation threshold. The frequencies occurringbetween ω_(ξ) and ω_(MWS) indicate the parallel nature of the behaviourof the real and imaginary permittivity components, as shown, forexample, in FIG. 8.

Thus, as the percolation threshold is approached, the lowercharacteristic frequency ω_(ξ) tends to zero.

This discussion highlights the importance of an accurate quantitativedescription of the electromagnetic response of materials near thepercolation threshold to the design of composite materials forelectromagnetic applications. The inventor has appreciated that theexisting theoretical models are wrong. The Maxwell-Garnet model (seeFIGS. 4 and 5) is both quantitatively and qualitatively wrong in that itentirely fails to predict percolation threshold effects. The Bruggemanmodel (see FIGS. 6 to 9) is quantitatively wrong as although it predictspercolation effects it predicts values for the percolation thresholdwhich differ widely from actual measured values.

FIGS. 9 a to 9 f present the generic regimes according to the Bruggemanmodel for the frequency dependence of the electromagnetic properties ofcomposites for filler volume fractions below, at and above thepercolation threshold. The concentrations used are (V_(c)−0.70),(V_(c)−0.01), Vc, (V_(c)+0.01) and (V_(c)+0.70), (all volumeconcentrations) where V_(c) is the critical filler volume fractioncorresponding to the percolation threshold. FIG. 9 a shows the realpermittivity, FIG. 9 b the imaginary permittivity, FIG. 9 c theconductivity, FIG. 9 d the dielectric loss tangent, FIG. 9 e the realelectric modulus and FIG. 9 f the imaginary electric modulus. It isobserved that a metallic or plasma-like dielectric response is notpredicted even for filler concentrations well above the percolationthreshold.

As discussed above, the inventor has however appreciated that theexisting theoretical models are flawed. The inventor is the first toappreciate that mixtures of conductive and non-conductive parties canexhibit a dielectric response at conductive filler concentrations fromnear to and above the percolation threshold.

In the light of the inventor's realisation, a series of experiments todetermine the feasibility of producing composite materials which exhibita plasma frequency and a negative permittivity to incident radiation ofselected frequencies or ranges or frequencies were carried out.

Initially, experiments were carried out to determine the percolationthreshold of each type of conductor-insulator composite (defined by aunique choice of conducting filler and insulating host medium) and todetermine the level of conductivity achieved in composites with fillerconcentrations above the percolation threshold. Such experiments wouldalso determine whether the percolation threshold and the dielectricproperties of the materials were influenced by any particle size effects(for example the ratio of the conducting particle size to the insulatingparticle size).

For these experiments, composites comprising mixtures of small (relativeto the effective wavelength of electromagnetic waves in the composite)particles of conductive materials such as metals or conductive ceramicsand small particles of insulating materials such as insulating polymersare made up by mixing controlled quantities of the conductive andinsulating particles to form a loose powder mixture. The materials maybe mixed using a shaker mixer and the particles may be of any suitableaverage size or size distribution, although particle sizes that aresmall (less than one tenth) of the wavelength of interest are preferred.

In particular, where the selected frequencies are in the range 0.1 to100 GHz (i.e. wavelength in the range 3 m to 3 mm), suitable particlesize distributions are from 1 nm to 250 nm for the conductive particles(for example, nano-silver, having an average particle size of 10 nm) and1 μm to 100 μm for the non-conductive particles. The powder mixture wasthen die pressed at room temperature to provide a consolidated compositemedium, for example using a pressure in the range of 130-260 MPa appliedfor a period in the range 60-300 seconds.

The plasma frequencies determined in the following experiments give riseto a range of effective wavelengths within the actual material. Thevalues of these effective wavelengths are determined using theequations:

$\begin{matrix}\left. {{\left. {{C = {f\; \lambda}};{C = {{\sqrt[1]{\left( ɛ \right.}}_{r}\mu_{r}}}} \right) \cdot {\sqrt[1]{\left( ɛ \right.}}_{o}}\mu_{o}} \right) & (20)\end{matrix}$

where ∈_(r) and μ_(r) are the relative permittivity and relativepermeability respectively, ∈_(o) and μ_(o) are the permittivity andpermeability in a vacuum and c is the speed of light.

Initially, experiments were carried out to study the dielectricproperties of composite materials comprising various fillers andconductive components. In each of these experiments, the conductivecomponents are in the form of particles. The non-conductive componentsmay also be composed of particles.

Size measurements for very small particles are dependent on the form ofmeasurement used to analyse the particles. This is because of bothmorphology effects being important and the fact that the particles willbe polydisperse (not all of the same size). In the following experiments(and elsewhere in this patent application), sizes are average sizesdetermined by specific surface area measurements (BET).

Experiment 1 (See FIGS. 10 a to 10 d)

Initially, four nano-aluminium PTFE (polytetrafluoroethylene) mixtureswere prepared, with two different PTFE average particle sizes used toinvestigate particle size effects, as shown in Table 1 below. Thenano-aluminium had an average size of 100 nm as measured using specificsurface area measurements (BET). The two other experiments and thepreferred embodiments of the invention described above PTFE particlesizes used in this, were 1 micron powder (Aldrich 43093-5) and 100micron powder (Aldrich 46811-8).

TABLE 1 nano-aluminium and PTFE particle sizes in initial experimentnano-aluminium PTFE particle size concentration (vol. %) (μm) 1.7 1008.1 100 8.1 1 15.6 1

For each composition, appropriate quantities of the different materialswere measured into a container. The container was then placed in a dryargon atmosphere (less than 50 ppm air) for at least 12 hours to removeany residual moisture so as to reduce particle agglomeration duringmixing. The container was then sealed under the argon atmosphere beforeplacing on a shaker mixer that was then operated for approximately 60minutes to thoroughly mix the particles. The argon atmosphere minimisesany further oxidation of the particles during mixing. The resultingpowder was then die-pressed at room temperature at a pressure of 260 MPafor 300 seconds to produce test samples.

For the measurements of complex permittivity over the frequency range 10mHz to 1 GHz the sample geometry was a disc with a diameter of 10 mm anda uniform thickness in the range 0.5 to 5.0 mm. The top and bottom facesof the sample were coated with a conducting paint to improve electrodecontact. For measurements of complex permittivity and permeability overthe frequency range 0.5 to 18 GHz the sample geometry was a toroid withan outer diameter of 6.995 mm and an inner diameter of 3.045 mm(designed to fit standard 7 mm coaxial microwave transmission line). Thesamples again had a uniform thickness in the range 0.5 to 5.0 mm.

The resulting composite was then subjected to a number of experiments todetermine its frequency dependent dielectric properties and itsstructure.

Electrical properties of the composites of experiment 1 are shown inFIGS. 10 a to 10 d. FIG. 10 a illustrates the real permittivity, FIG. 10b the conductivity, FIG. 10 c the dielectric loss tangent and FIG. 10 dthe imaginary electric modulus for nano-aluminium dispersed in PTFE.These measurements were undertaken at room temperature using aNovocontrol broadband dielectric spectrometer, comprising a NovocontrolAlpha dielectric analyser for the frequency range up to 1 MHz and anAgilent 4291 RF Impedance analyser for the frequency range 1 MHz to 1GHz.

A comparison of FIG. 10 to FIG. 9, suggests that the highest aluminiumconcentration for each PTFE particle size is above the percolationthreshold, as the trends in FIG. 10 in real permittivity, conductivity,dielectric loss and electric modulus are similar to those forcompositions in FIG. 9 which are above V_(c). In addition, it isfeasible that the percolation threshold for the larger PTFE particlesize is lower. Therefore, it is surprising that the increase inconductivity at 10 mHz from the lowest to highest aluminiumconcentration for a given PTFE particle size is less than three ordersof magnitude. Normally, for composites containing metal fillerparticles, it is expected that the percolation transition would resultin at least ten orders of magnitude increase in composite conductivityat such a frequency. Moreover, for filler concentrations above thepercolation threshold, the composite conductivity would exceed 1 S/m. Inaddition, the upper limiting frequency, ω_(MWS), for maximum dielectricloss appears several orders of magnitude below the microwave frequencyrange, (for comparison, conventional metal particles yield valuesseveral orders of magnitude above 1 GHz). This reduction in ω_(MWS)suggests that there has been a significant reduction in the conductivityof the conducting phase, below that of bulk aluminium. This may be dueto appreciable surface oxidation of the aluminium nano-particles. Thisoxidation may be due in part to the particles being supplied under air,rather than under hexane, which is known to prevent or at least reducesurface oxidation effects. Because the resulting composite conductivitywas so low and the upper characteristic frequency for critical behaviourassociated with percolation theory was deduced to be below the microwaveregion, microwave measurements of the complex permittivity andpermeability were not undertaken.

Experiment 2 (See FIGS. 11 a to 14 g)

Eight different silver/PTFE composites were prepared. Silver particleswith a mean size of approximately 100 nm were dry-mixed with PTFE(polytetrafluoroethylene) particles as shown in Table 2:

TABLE 2 nano-silver and PTFE particle sizes in initial experiment PTFEaverage size PTFE average size 100 μm 1 μm nano-silver 0.5 1concentration 1 2 (vol. %) 5 10 15 20

Composites were prepared as described for Experiment 1. The resultingcomposite was then subjected to a number of experiments to determine itsfrequency dependent dielectric properties and its structure.

The electrical properties of the composites resulting from differentconcentrations or fractions of silver in 100 μm PTFE are shown in FIGS.11 a to 11 d. FIG. 11 a illustrates the real permittivity, FIG. 11 b theconductivity, FIG. 11 c the dielectric loss tangent and FIG. 11 d theimaginary electric modulus for nano-silver dispersed in 100 μm PTFE.

The electrical properties of the composites resulting from differentfractions of silver in 1:m PTFE are shown in FIG. 12 a to 12 d. FIG. 12a illustrates the real permittivity, FIG. 12 b the conductivity, FIG. 12c the dielectric loss tangent and FIG. 12 d the imaginary electricmodulus for nano-silver dispersed in 1 μm PTFE. The measurements shownin FIGS. 11 a-11 d were undertaken at room temperature using aNovocontrol broadband dielectric spectrometer, comprising a NovocontrolAlpha dielectric analyser for the frequency range up to 1 MHz and anAgilent 4291 RF Impedance analyser for the frequency range 1 MHz to 1GHz.

The nano-silver composites exhibited a more obvious percolative responsethan the nano-aluminium composite, with the higher silver concentrationsresulting in composites with significant conductivity for both PTFEparticle sizes. There is also greater qualitative evidence that thepercolation threshold is lower for a larger PTFE particle size, with thepercolation threshold lying between 1.0 and 5.0 vol % for 100 μm PTFE,and between 2.0 and 10.0 vol. % for 1 μm PTFE. Given that the resultsfor 1.0 and 2.0 vol. % for 1 μm PTFE are quantitatively very similar, itwould appear that the percolation threshold will be significantly above2.0 vol. %.

FIGS. 13 and 14 a to g show the microwave response for the samplesprepared in Experiment 2. These measurements were made using an Agilent8510 Vector Network Analyser with an S-parameter Test Set and 7 mmCoaxial Transmission Line according to the method of Nicolson, Ross(IEEE Trans Instrum. And Meas., vol 19, p 377, 1970) and Weir (Proc.IEEE, vol 62, p 33, 1974).

It is observed that for silver concentrations above the percolationthreshold, some samples have a real permittivity whose frequencydependence is unlike that expected from the Bruggeman model (throughcomparison to FIG. 9 a, see samples XC02379 and XC02380 atconcentrations of 5% and 15% in FIGS. 13 c and 13 d). The measuredfrequency dependence closely resembles that expected for a plasma. Sometest samples exhibit a plasma frequency in the measured frequency range.Other test samples have a plasma frequency above the measured frequencyrange. For the 1 μm PTFE samples, some samples only have a real positivepermittivity, which is a typical response for conductive compositematerials. The plasma-like response is most consistently observed forthe 100 μm PTFE samples. The conductivity highlights the percolationtransition, as shown in FIGS. 13 e and 14 e. These microwave responseresults indicate that the material would be reflective to incidentelectromagnetic radiation at frequencies below the plasma frequency, butstrongly absorbing above it.

These measurements also indicate a diamagnetic effect for silverconcentrations above the percolation threshold, with a maximum magneticloss associated with this effect. This is consistent with theKramers-Kronig relations. Visual inspection of the composite materialhighlighted a significant optical reflectivity and a silvery appearance.

FIG. 14 h compares the filler concentration dependence of theconductivity for different silver particle filled composites at anarbitrary frequency of 0.5 GHz. Composites formed from nano-silverparticles dispersed with 100 μm and 1 μm PTFE particles are compared topreviously obtained silver coated microspheres dispersed in paraffin wax[see Youngs I. Dielectric measurements and analysis for the design ofconductor/insulator artificial dielectrics. IEE Proc., Sci. Meas. &Tech., 147(4), p 202, July 2000; Youngs I. Electrical percolation andthe design of functional electromagnetic materials. PhD Thesis,University College, London. 2001]. It is observed that the gradient ofthe percolation transition for the nano-silver/1 μm PTFE composite issimilar to that for the microsphere/wax composites although the latterhas a higher percolation threshold. In contrast, the gradient of thepercolation transition for the nano-silver/100 μm PTFE composites ismuch reduced. This difference is consistent with the relative positionsof the composites on the particle size ratio scale. The microsphere/waxsystem exhibits a perfectly random microstructure and because theparticle size ratio of the nano-silver/1 μm PTFE system is relativelyclose to unity its microstructure should be similarly random, whereasthe nano-silver/100 μm PTFE system exhibits a clear excluded-volumemicrostructure. This striking difference serves to explain the increasedrepeatability observed in the properties of nominally identical samplesof nano-silver/100 μm PTFE prepared at filler concentrations spanningthe transition region.

As can be seen in FIG. 14 h (which shows samples exhibiting aplasma-like response as solid data points and/or data points with abackground) the plasma like response is exhibited for samples above thepercolation threshold and on or approaching the upper plateau of theconductivity against concentration plot. The experiments suggest thatthe composite must have a conductivity of greater than 10 S/m andpreferably about 30 S/m for a plasma-like response to be exhibited.

Experiment 3 (See FIGS. 15 a and 15 b)

Titanium diboride powder, of a maximum particle size of 45 μm wasdry-mixed with PTFE particles having an average size of 1 μm at atitanium diboride fraction of 50 vol. %, and processed as describedabove for Experiment 1. The titanium diboride powder was 45 micronpowder purchased from Goodfellow Cambridge Limited.

FIGS. 15 a and 15 b, respectively, show the experimental complexpermittivity and permeability spectrum of the resulting composite, overa frequency range of 0.5 to 18 GHz (measured using the same method usedin Experiment 2.

Titanium diboride was selected because it is an oxidation resistantceramic conductor.

The plasma resonance ω_(p) is clearly visible at approximately 3 GHz.There are additional zero-points in the real permittivity (atapproximately 5 and 10 GHz), unlike the silver samples discussed above.The highest (3rd) zero crossing (shown as ω_(p1)) is a plasma frequencythat may be associated with a group of charge carriers that are morelocalised (which cannot cross the sample and so are probably part offinite clusters unconnected with the percolating cluster). Reference canbe made to the Handbook of Conducting Polymers (Kohlman R et al ISBN0-8247-0050-3). The ratio of ω_(p) to ω_(p1) is associated with theratio of free electrons to the full conduction electron density.

There were difficulties in replicating the results of Experiment 3. Theinventor believes that these difficulties may result from the fact thatthe conductive titanium diboride particles are larger than thenon-conductive PTFE particles.

Experiments 4 and 5 (See FIGS. 16 to 18)

Following the results of experiments 1 to 3, the inventor hasappreciated that it is also possible to produce composite materialsutilising copper and cobalt nano-particles. Three composite materialswere made: a nano copper in PTFE composite comprising copper particleshaving an average size of 90 nm and PTFE particles having an averagesize of 100 μm; a nano cobalt in PTFE composite with cobalt particleshaving an average size of 20 nm and PTFE particles having an averagesize of 100 μm; and a nano cobalt in wax composite with cobalt particleshaving an average size of 20 nm. The materials were produced asincluding PTFE and all the experiments carried out as described abovefor Experiment 1. The cobalt-wax composites were prepared by firstdissolving the required quantity of paraffin wax (paraffin waxflakes—Aldrich 41166-3) using hexane and then stirring-in the requiredquantity of nano-cobalt powder. Stirring was continued until the solventevaporated and a solid mixture remains. Test samples were prepared bydie-pressing as described for Experiment 1.

FIGS. 16 and 17 show the measured dielectric responses for the copperand cobalt composites, respectively, the experiments 4 and 5. Althoughthe dielectric responses of copper and cobalt are similar to that ofaluminium, as shown in FIGS. 16 and 17, of these three fillers, cobaltcomposites produce the highest conductivity, subject to the accuracy offiller concentration. FIGS. 16 a and 17 a show real permittivity, FIGS.16 b and 17 b show imaginary permittivity, FIGS. 16 c and 17 c showconductivity, FIGS. 16 d and 17 d show dielectric loss tangent, FIGS. 16e and 17 e show real electric modulus and FIGS. 16 f and 17 f showimaginary electric modulus.

FIG. 18 shows that negative real permeability has not been observed ineither cobalt-PTFE or cobalt-wax composites, but that a ferromagneticcontribution (the reduction in real permeability with increasingfrequency) inherent to the cobalt particles is observed.

Cobalt is a transition metal with unpaired electrons in the outerd-orbitals. These unpaired electrons give rise to domains of alignedmagnetic dipoles and a net magnetisation which may be represented by avector precessing about a preferred crystallographic axis. Theprecession frequency is determined by specific material parameters whichrelate to the magnetic anisotropy field inherent to the material. Anincident electromagnetic wave can couple to this precession and at acritical frequency at which the incident frequency approaches thenatural precession frequency resonant absorption will occur. For thetransition metals and many ferrites (transition metal oxides) thisoccurs at microwave frequencies. The features observed in theexperimental data are evidence of this process and moreover, demonstratethat damping processes are present resulting in features that are closerto the relaxation form (discussed for dielectric response) rather than asharp resonance.

This ferromagnetic contribution increases with filler fraction, althoughthe dependence of the magnetic properties on the filler fraction is notdependent on the percolation threshold. Consequently, it is possible tomaximise the magnetic properties by simply increasing the fillerfraction or concentration.

In composites embodying the present invention (including those discussedin relation to the experiments FIGS. 10-18 above); the electricallyconductive material exhibits no long range order over a distance of theorder of the wavelength of radiation propagating in the material, andfor frequencies close to the plasma frequencies (where the permittivitywould be close to zero and there is a singularity), the effectivewavelength of electromagnetic radiation in the material diverges. Wavestravelling through a material have an effective wavelength which isgoverned by the permittivity of the material. As the material'spermittivity drops, the effective wavelength increases. However, thereis a singularity because at the plasma frequency the permittivity iszero which would give an effective wavelength of infinity.

This should not be taken to mean that amongst the conductive componentthere is no regular ordering of individual particles, but merely thatclusters and networks are formed. In the composites, the conductivematerial is randomly dispersed although not necessarily uniformlydispersed. There is no form of periodicity in the dispersion of theconductive component. The amount of electrically conductive material ispreferably sufficient to form a conductive network, extending over adistance of the order of the effective wavelength of radiationtravelling through the material. There is therefore also no long rangeorder of particles forming the network or within the network.

A single conductive network may be formed, which extends from one faceof the material to another, preferably an opposite face, or a pluralityof linked networks (i.e. linked by clusters) may be formed.

The network may be in one, two or three dimensions. This merely reflectsthe dimensionality of the connectivity between the individual elementsforming the network. However, this does not place any form of limitationon the structure or design of the material in which the network exists.For example, it may be possible to have a three-dimensional material,which contains a two-dimensional network. Other forms of material, suchas sheets or hollow bodies manufactured from sheets or other materialsmay also contain one-dimensional, two-dimensional or three-dimensionalnetworks.

Although only materials which are designed to exhibit a negativepermittivity with a plasma frequency in the microwave regions of theelectromagnetic spectrum have been described here, it will be understoodby those skilled in the art that the same techniques of materials designand production can be applied to produce a composite material whichexhibits a small positive permittivity, resulting in a material with asmall (less than unity) positive refractive index. Such materials are ofinterest as if their refractive index is less than that of air, totalinternal reflection could be achieved easily for radiation incident fromair onto such a material.

The physics underlying the effects described above is complicated andnot yet fully understood. As is clear from the experiments carried outby the inventor the existing models fail to accurately predict thebehaviour of composite materials having conductive material in aninsulating host. The inventor was the first to appreciate how suchmaterials would behave and how they have a plasma frequency which may beaffected by the nature of the electrically conductive and non-conductivematerials making up a composite material. The inventor's analysissuggests that there are a number of theoretical models which whenmodified, the inventor believes have the potential to fit theexperimental evidence and explain the dependence of the plasma frequencyon material parameters such as particle shape, size, conductivity,microstructure and concentration to aid composite design.

The candidate models identified by the inventor as having the potential,when modified, to fit the measured microwave plasma-like responseinclude:

1) The model for the infra-red dielectric response of intrinsicallyconducting polymers discussed in Kohlman R, Epstein A. Insulator-metaltransition and inhomogeneous metallic state in conducting polymers.Chapter 3 (pages 100-110 in particular) in Handbook of ConductingPolymers, 2^(nd) Ed., Marcel Dekker, New York, 1998;

2) The model for metallic patches joined by narrow connections discussedin Govorov A, Studenikin S, Frank W. Low frequency plasmons in coupledelectronic microstructures. Physics of the Solid State, 40(3), p 499,1998; and

3) The effective medium model discussed in Sarychev & Shalaev. EMproperties of metal-dielectric composites beyond the Quasi-staticapproximation. Physics Reports, 335, p 275 371 2000.

A comparison of the inventor's experimental results described herein,appears to indicate that a modified version of the Sarychev and Shalaevmodel provides a qualitative match to the experimental data. This is aneffective medium model that goes beyond the quasi-static approximationby including a skin-depth component (to determine the extent to whichapplied fields die away within the material)

$\begin{matrix}{{{{\left( {1 - V} \right)\frac{ɛ - ɛ_{m}}{{2\; ɛ} + ɛ_{m}}} + {V\frac{ɛ - {\overset{\sim}{ɛ}}_{f}}{{2\; ɛ} + {\overset{\sim}{ɛ}}_{f}}}} = 0}{with}} & \left( {21\; a} \right) \\{{\overset{\sim}{ɛ}}_{f} = {ɛ_{f}\frac{2\; {F\left( {k_{f}a} \right)}}{1 - {F\left( {k_{f}a} \right)}}}} & \left( {21\; b} \right) \\{{{F(x)} = {\frac{1}{x^{2}} - \frac{\cot (x)}{x}}}{and}} & \left( {21\; c} \right) \\{k_{f} = {\frac{2\; \pi \; f}{c}\sqrt{ɛ_{f}\mu_{f}}}} & \left( {21\; d} \right)\end{matrix}$

By inspection, it is deduced that this model is an extension of thesymmetric Bruggeman model given earlier. McLachlan (McLachlan D, HeissW, Chiteme C and Wu J. Physical Review B, 58(20), p 13558, 1998.) haspreviously modified the Bruggeman model to introduce the features ofpercolation theory in a more quantitative fashion. Specifically,McLachlan introduces the percolation threshold and the power lawexponents

$\begin{matrix}{{{\left( {1 - V} \right)\frac{ɛ^{1/s} - ɛ_{m}^{1/s}}{{\left( \frac{1 - V_{c}}{V_{c}} \right)ɛ^{1/s}} + ɛ_{m}^{1/s}}} + {V\frac{ɛ^{1/t} - ɛ_{f}^{1/t}}{{\left( \frac{1 - V_{c}}{V_{c}} \right)ɛ^{1/t}} + ɛ_{f}^{1/t}}}} = 0} & (22)\end{matrix}$

The similarity of these models leads to the application, by theinventor, of McLachlan's phenomenological modifications to theSarychev-Shalaev model,

$\begin{matrix}{{{\left( {1 - V} \right)\frac{ɛ^{1/s} - ɛ_{m}^{1/s}}{{\left( \frac{1 - V_{c}}{V_{c}} \right)ɛ^{1/s}} + ɛ_{m}^{1/s}}} + {V\frac{ɛ^{1/t} - {\overset{\sim}{ɛ}}_{f}^{1/t}}{{\left( \frac{1 - V_{c}}{V_{c}} \right)ɛ^{1/t}} + {\overset{\sim}{ɛ}}_{f}^{1/t}}}} = 0} & (23)\end{matrix}$

Analogous equations can be set out for the magnetic permeability.

The real benefit of the new model is that it can be used tosimultaneously predict or fit both the complex permittivity andpermeability of a conductor-insulator composite. The parameters in themodel are:

-   -   matrix and filler permeability and permittivity, complex if        required;    -   filler concentration or fraction;    -   percolation threshold;    -   percolation exponents;    -   filler particle size; and    -   frequency of the applied electromagnetic field.

FIG. 19 illustrates an attempt to fit representative experimental data,in the form of the complex permittivity and permeability for 5 vol. %silver nano-particles (the average size 100 nm) mixed with 100 μm PTFEparticles, over the frequency range 0.5 to 18 GHz using theSarychev-Shalaev-McLachlan model. In this case, the percolationexponents were set at unity, representing the situation for theSarychev-Shalaev model. All other parameters were set to valuesrepresentative of the measured composite as shown in Table 3:

TABLE 3 Parameters for FIG. 19 Parameter Value Matrix permittivity2.1-j0.001 Matrix permeability 1 Filler conductivity (S/m) 1E7 Fillerpermeability 1 Percolation threshold 0.04469, 0.04470 Filler volumefraction 0.05 Percolation exponent, s 1.0 Percolation exponent, t 1.0Filler particle radius (nm) 50

It is observed that the diamagnetic effect in the magnetic permeabilityis not predicted, the conductivity of the composite is over estimatedand no minimum is predicted, but most significantly a plasma frequencyis not predicted even with control of the percolation threshold to atolerance of 0.001 vol. %.

If the values of the percolation exponents are set to the universalvalues for a three-dimensionally connected network, then it becomespossible to predict a plasma-like response. This is illustrated in FIG.20. However, the gradient of the real permittivity at the plasmafrequency remains poorly predicted, as does the composite conductivityand the magnetic permeability. The parameters used in this calculationare shown in Table 4:

TABLE 4 Parameters for FIG. 20 Parameter Value Matrix permittivity2.1-j0.001 Matrix permeability 1 Filler conductivity (s/m) 1E7 Fillerpermeability 1 Percolation threshold 0.04 Filler volume fraction 0.05Percolation exponent, s 0.73 Percolation exponent, t 1.9 Filler particleradius (nm) 50

A much better qualitative fit to all four parameters is obtained byre-considering the structure of the composite. In the case of thenano-silver particles mixed with 100 μm PTFE particles, concentrationsabove the percolation threshold resembled a close-packed arrangement ofapproximately 100 μm diameter pseudo-conducting particles. Thepseudo-conducting particles are taken to have a PTFE core withsemi-continuous or continuous silver coating created by the silvernano-particles. This is shown in the SEM (scanning electron microscope)images of FIG. 21.

FIGS. 21 a, 21 b, 21 c and 21 d show backscattered images ofcompositions comprising 0.5 vol %, 1.0 vol %, 5.0 vol % and 15 vol %nano-silver particles and 100 μm PTFE particles respectively. In FIGS.21 a and 21 b, it is clear that individual silver particles form someclusters on the surface of the PTFE particles, but not enough to form aconductive network. Consequently these particular samples do notconduct, or exhibit a plasma frequency.

FIGS. 21 c and 21 d show compositions with a higher nano-silverconcentration. In FIG. 21 c, the nano-silver concentration is highenough that some clusters have begun to form networks, one of which isshown stretching from the left-hand side of the image to the right-handside. In FIG. 21 d, the silver concentration is high enough to form acoating of approximately three silver particles deep over each PTFEparticle. Both of the samples shown in FIGS. 21 c and 21 d conduct, andexhibit a plasma frequency.

FIGS. 21 e and 21 f show materials with identical nano-silverconcentrations (5.0 vol %) with PTFE particles of 100 μm and 1 μm size,respectively. The nano-silver distribution in FIG. 21 f is fairlyregular across the entire sample, whereas that in FIG. 21 e clearlyforms a network.

FIGS. 21 g and 21 h show backscattered images of two nominally identicalcompositions with 10 vol % nano-silver particles and 1 μm PTFEparticles. The sample in FIG. 21 g exhibited a plasma frequency, whereasthat in FIG. 21 h, did not, but exhibited a “conventional” positivepermittivity.

It is necessary to determine how the model parameters relate to thematerials tested, which is determined by the behaviour of the insulatorphase, the PTFE particles. Taking a case where the PTFE particles have anominal radius of 50 μm, the silver particles have a tendency to coatthe surface of the PTFE particles. Ultimately, this leads to thecreation of pseudo-conducting particles once there is a percolatingnetwork of silver particles over the PTFE particle surface. This hasoccurred in the samples tested because the results demonstrate asignificant DC conductivity. These conductor-coated particles are alsoclose-packed. Close-packing occurs for concentrations of the order of 60vol %.

A second explanation is that the properties are driven bytwo-dimensional percolation over the sample surface because thetheoretical percolation threshold for two-dimensional systems is 50 vol%. These points are emphasised by the backscatter scanning electronmicrographs presented in FIGS. 21 a to 21 d.

It is also of interest to compare the microstructures of the compositesformed using 100 μm and 1 μm PTFE, and to consider why the properties ofthe latter have a much lower sample to sample repeatability, as shown inFIGS. 21 e and 21 f. The excluded volume microstructure is much lessevident for the smaller PTFE particle size. In fact, in this case, thedistribution of silver particles appears much closer to a distributionthat might be formed if the silver particles are allowed to occupy spacein the composite on a perfectly random basis. The issue of repeatabilitycan be explained as follows. When the conducting filler particles areable to fill space on a perfectly random basis, then a composite samplewill only become conductive when there is a connected network ofconducting particles across the bulk of the sample. However, when theinsulating matrix particles are much larger than the filler particles,the bulk sample will conduct when there is a percolated layer ofparticles surrounding individual matrix particles. Simplistically, thescale of control is reduced to an individual particle surface ratherthan the bulk dimensions of the object. At present, the gradient of thetransition from the excluded-volume dominated behaviour to the randomfilling behaviour, as a function of particle size ratio, is not known.The steeper this transition, the smaller the matrix particles can bewithout reducing repeatability. This would lead to the prospect ofthinner coatings or smaller components.

Since the conducting filler distribution is critical to the phenomenon,it is also interesting to compare the microstructures for two nominallyidentical samples, but which give quite different dielectric response.For example, FIGS. 21 g and 21 h compare two samples, which arenominally 10 vol % concentrations of silver nano-particles dispersedwith 1 μm PTFE particles. The sample shown in FIG. 21 g exhibited amicrowave plasma frequency, whereas that shown in FIG. 21 h had aconventional positive dielectric response. The micrographs reveal asubtle difference in silver particle distribution. There is anindication that the silver particles are more uniformly dispersed in thesample shown in FIG. 21 g. In the context of the model, a uniformdispersion of sufficient filler particles to form a percolation patharound a matrix particle should more readily enable percolation over thebulk and a higher composite conductivity. This is consistent with theexperimental conductivity data. The conductivity for high silverconcentrations in the 100 μm PTFE composites is much more repeatable andat the higher end of the spread in the equivalent data for the 1 μm PTFEcomposites. It is the 1 μm PTFE samples with highest conductivity thatexhibit the plasma response. This observation further supports thehypothesis that there is a critical conductivity that must also besurpassed to achieve the plasma response. This critical conductivitycould be associated with a conducting material being classed as ‘trulymetallic’. Indeed, the critical conductivity deduced from the availabledata is close to Mott's limiting value for metals (approximately 10⁴S/m). To further put this into context, the conductivity of bulk copperis approximately 10⁸ S/m.

Consequently, it may be relevant to re-assign different values to theconducting filler concentration, the percolation threshold and thefiller particle size. The resulting fit is illustrated in FIG. 22. Theparameters used in this calculation are given in Table 5 below:

TABLE 5 Parameters for FIG. 22 Parameter Value Matrix permittivity2.1-j0.001 Matrix permeability 1 Filler conductivity (S/m) 1E7 Fillerpermeability 1 Percolation threshold 0.6 Filler volume fraction 0.6025Percolation exponent, s 0.73 Percolation exponent, t 1.9 Filler particleradius (nm) 50,000

As can be seen from the modelling results (in FIGS. 19, 20 and 22),although a good qualitative fit is obtained, there are somediscrepancies where differing sizes of PTFE filler are used. Theexperiments show that there is little difference in the magnitude of thediamagnetic effect for samples with 100 μm PTFE particles or 1 μm PTFEparticles. In the model, diamagnetic effect is partly compensated for byadjusting particle size. Consequently, the predicted properties forsmall particle composites differ somewhat from those observed inexperiments. However, it may be possible to overcome this by modellingthe diamagnetic effect by including a macroscopic toroidal fieldcomponent, or alternatively using Mie theory, although other factorssuch as sample geometry must be taken into account.

FIG. 22 demonstrates that a good qualitative fit can be obtained usingthe modified Sarychev-Shalaev-McLachlan model for the 5 vol. % silvernano-particles mixed with 100 μm PTFE particles, albeit after somere-assignment of certain parameters including the filler particle size,filler fraction and percolation threshold. For such modifications to betruly permissible, then they should hold for related cases. An importantexample, is the 10 vol % silver nano-particles mixed with 1 μm PTFEparticles. Here, the adjusted filler particle radius would need to be500 nm. This would have the effect of significantly reducing thediamagnetic effect in the microwave range.

However, comparison of FIGS. 13 f and g to FIGS. 14 f and g indicatethat the diamagnetic effect is largely unaffected by the change in PTFEparticle size. Thus, greater understanding is required before themodified Sarychev-Shalaev-McLachlan model can be used to quantitativelydesign materials of this type.

The inventor has also observed plasma-like frequencies at much lowerfrequencies, as shown in FIGS. 23 a and 23 b. Materials with anano-silver concentration of 5 vol %, and a PTFE particle size of 100 μmdemonstrate a conductivity change at 10⁴ Hz (FIG. 23 a), and a negativereal permittivity at around 10³ Hz (FIG. 23 b). These materials wereprepared in the manner discussed above for Experiment 1. In each case,the samples were cooled to −60° C. and −10° C. or heated to 30° C. Thisgave fairly consistent results, with one sample exhibitingrepeatability.

The issues of particle size, particle packing and contact areas of theparticles in the composite material have been explored further by theinventors in order to understand the mechanism by which the conductivitygradient changes, and to enable the production of materials of uniformand repeatable compositions having tailored dielectric and conductiveproperties. The materials comprise regions of electrically conductiveand non-electrically conductive materials, where the conductivity ofeach material is determined by the degree of connectivity between theelectrically conductive regions.

FIG. 25 compares the concentration dependence of the conductivity offour compositions at 0.5 GHz:

Ag (100 nm particle size) and PTFE (1 μm particle size);Ag (100 nm particle size) and PTFE (1 μm particle size);Ag (100 nm particle size) and paraffin wax; andAg (15 μm diameter spheres) and paraffin wax.

For each composition, appropriate quantities of the different materialswere measured into a container. The container was then placed in a dryargon atmosphere (less than 50 ppm air) for at least 12 hours to removeany residual moisture to reduce particle agglomeration during mixing.The container was then sealed under the argon atmosphere before placingon a shaker mixer that was then operated for approximately 60 minutes tothoroughly mix the particles. The argon atmosphere minimises any furtheroxidation of the particles during mixing. The resulting powder was thendie-pressed at room temperature at a pressure of 260 MPa for 300 secondsto produce test samples.

The behaviour of these materials in the region of the percolationthreshold may be determined by either 3D percolation only at closepacking concentrations, or by 2D percolation over the surface of theinsulating particle. A distinction between these two types of behaviourcan be identified using the percolative power law exponents.

Although the gradients of the percolation transition for the 100 nm Ag/1μm PTFE composites is similar to that of microsphere/wax composites, thepercolation threshold of the microsphere/wax composites is higher. Thegradient of the percolation transition of the 100 nm Ag/100 μm PTFEcompositions is reduced, which is consistent with the relative positionsof the materials on a particle size ratio scale. The gradient (on alog-log scale) for the 1 μm PTFE material is approximately 30, whereasthat for the 100 μm PTFE material is approximately 7.

The microsphere/wax system exhibits a perfectly random microstructure,and the particle size ratio of the 100 nm Ag/1 μm PTFE is relativelyclose to unity (1:10), the microstructure is also similarly random.However, the 100 nm Ag/100 μm PTFE system has a particle size ratio of1:1000, and exhibits the properties of an excluded volumemicrostructure, whose physical properties arise from the use of a smallfiller concentration within a composite material. In an excluded volumemicrostructure, the regions of electrically conductive material will beexcluded from certain areas (the non-electrically conductive matrix),which means that in order for the material to exhibit an electricalconductivity, the conductive regions need to be connected somehow acrossthe non-electrically conductive regions. By increasing the number ofand/or volume of the excluded regions of the microstructure, the rate atwhich connections are formed for increasing concentrations of conductivematerial will drop, as it requires more material to connect over theexcluded regions than if there were few or smaller excluded regionspresent. This then produces the flattened gradient observed in theexperiments. It is also possible to use an electrically conductivematrix, such as a foam to produce a network around gas-filled pockets.This would also act as an excluded volume microstructure.

The power law exponents for the percolation transition can be determinedby scaling analysis of the real permittivity and conductivity of thecomposites discussed above for filler concentrations in the region ofthe percolation threshold. FIGS. 26 and 27 show the scaling of realpermittivity and conductivity respectively for 100 nm Ag/100 μm PTFEcompositions, and FIGS. 28 and 29 the scaling of real permittivity andconductivity respectively for 100 nm Ag.1 μm PTFE compositions.

According to percolation theory, these exponents should adopt universalvalues that only depend on the dimensionality of the percolationprocess. As the percolation threshold is approached from below, the realpermittivity should vary according to equation 24:

∈∝|v−v_(c)|^(−s)  (24)

with the exponent s taking the value of ≈0.73 for 3D systems and 1.33for 2D systems. Similarly, as the percolation threshold is approachedfrom above, the conductivity should vary in accordance with equation 25:

σ∝|v−v_(c)|^(t)  (25)

with the exponent t taking the value ≈1.9 for 3D systems and 1.33 for 2Dsystems. Table 6 below summarises the percolation threshold and exponentvalues obtained from this analysis, and includes the values determinedfor microsphere/wax composites, using the same technique, forcomparison.

TABLE 6 Composite type v_(c) s t Microsphere/wax 0.18 0.70 1.97 100 nmAg/1 μm PTFE 0.075 0.72 1.85 100 nm Ag/100 μm 0.0141 0.73 2.38 PTFE

The values of the exponents most closely resemble the universal valuesfor 3D systems, although the value of t for the 100 nm Ag/100 μm PTFEsystem is much larger than that of the 3D system. This is indicative ofa broader percolation transition.

According to percolation theory, power-law behaviour in the frequencydependence of the permittivity and conductivity is also expected forfiller concentrations near/in the transition region. The appropriatepower laws are given by equations 26 and 27:

∈′∝ω^(−γ()26)

σ∝ω^(x)  (27)

In the strictest sense, these power laws only apply at the percolationthreshold, but are often applied for filler concentrations near thethreshold. The values of these exponents are related to the exponents sand t within the context of a polarisation-based model. The actualrelationships are given in equations 28 and 29:

$\begin{matrix}{x = \frac{t}{s + t}} & (28) \\{y = \frac{s}{s + t}} & (29)\end{matrix}$

The relationship for both real and imaginary components is the same. For3D systems it is expected that x=0.72, y=0.28, and for 2D systems, thatx=y=0.5.

FIG. 30 presents the frequency dependent conductivity for a 2 vol % 100nm Ag/100 μm PTFE composite material over the frequency range 1 Hz to 1MHz. For composites well below the percolation threshold, theconductivity will be dominated by the capacitance between the conductingfiller particles and is therefore inversely proportional to frequency(having a gradient of −1). For composites well above the percolationthreshold and at low frequencies, the conductivity becomes dominated byconduction through connected conducting particles. The conductivitytherefore becomes frequency independent (having a gradient of zero). Thedata in FIG. 30 clearly shows an intermediate behaviour that isrepresented by a power law over the frequency range 1 kHz to 1 MHz (asshown by the trend line). The power-law exponent is shown to be 0.71,which is in good agreement with the expected value for a 3D system.However, this is not perfectly consistent with the non-universal valueof t derived from the concentration dependence.

FIG. 31 presents the corresponding data and power-law analysis for thereal and imaginary components of permittivity. The power-law exponentfor the imaginary permittivity is consistent. However, the power-lawcomponent for the real permittivity is not, which may indicate that thematerial tested had not quite reached the percolation threshold. If thepercolation threshold has been reached, the dielectric loss tangent (theratio of the real and imaginary permittivity components) is frequencyindependent, as predicted by percolation theory.

FIGS. 32 and 33 present an equivalent power law analysis of thefrequency dependence of the conductivity and permittivity of an 8 vol %100 nm Ag/1 μm PTFE material. This is a sample that has a fillerconcentration similarly related to the relevant percolation threshold,compared with the 2 vol % 100 nm Ag/100 μm PTFE sample discussed above.The data of FIG. 32 indicates that two distinct power-laws can be usedto describe the trend within the measured frequency range of 1 Hz to 1MHz. Over the frequency range 1 Hz to 1 kHz, the power-law exponent isin reasonable agreement with the expected value for a 3D system, asbefore. Again, the real component of the permittivity is not consistentwith the predicted and expected values.

The percolation behaviour therefore appears to be that of a 3D system,regardless of the particle size ratio of the conducting andnon-conducting components.

The frequency dependent dielectric properties of the composite materialexamined may also be interpreted using the “Universal DielectricResponse Theory” of Jonscher (Jonscher A., “The universal dielectricresponse and its physical significance”, IEEE Trans. ElectricalInsulation, 27(3), p 407, 1992, Jonscher A., “Dielectric relaxation insolids”, J. Phys. D: Appl. Phys., 32, pR57, 1999).

In materials in which the polarisation is dominated by slowly mobilecharge carriers, such as those whose mobility is dominated by hopping,the loss peaks due to relaxation of such a polarisation process arereplaced by a fractional power-law or constant phase angle responsegiven by equation 30 and illustrated in FIG. 34:

∈″(ω)/∈′(ω)=cot(nπ/2)  (30)

The extreme low frequency dispersion (LFD) is due to the fact that thecharges are relatively unbound and can move over large distancescompared to more conventional dipoles that give rise to a dielectricresponse due to polarisation effects. Moreover, whilst these charges arerelatively free to move, a dc conductivity, indicated by a frequencyindependent real permittivity is not observed. The general responseshown in FIG. 34 can be compared to the experimental data in FIGS. 31and 33. There is a clear correspondence between FIGS. 31 and 34,including the crossover of the real and imaginary traces. Thiscomparison may provide an explanation for the inconsistency between thepower-law exponents derived from the data in FIGS. 31 and 33. In bothfigures, there is no constant ratio between the real and imaginarycomponents. This is indicative of the crossover region. The crossoverrange therefore occurs at frequencies outside of those measured.

The repeatability of the observed properties of the above compositematerials was also investigated by the inventors. In particular, therepeatability of a plasma-like response (where the material acts as ifit is a metal, exhibiting a plasma frequency) when the particle sizeratio increases, was investigated.

FIG. 35 summarises the experimental results in terms of the measuredconductivity at 0.5 GHz, with the error bars representing the spread ofresults from 3 nominally identical samples. Although there is no clearindication that the reproducibility varies with size ratio, there is anindication that the size ratio affects the gradient of the percolationtransition. This is important for ensuring the reliability ofcompositions prepared within or sufficiently near the transition region.

Inter-particle contact resistance and therefore contact area areimportant factors in determining the overall conductivity of thecomposites. Dielectric measurements were taken to examine the conductionmechanism. These measurements were undertaken using a Novocontrol AlphaDielectric Spectrometer and Novocontrol Quatro Cryosystem. Dielectricspectra over the frequency range 1-10⁷ Hz were collected fortemperatures over the range −150 to 50° C. at 10° C. intervals. Somefurther measurements were repeated over the temperature range −100° C.to 100° C. at 5° C. intervals.

The following samples were tested:FIG. 36: 1 vol % 100 nm Ag in 100 μm PTFE (samples B, C);FIG. 37: 2 vol % 100 nm Ag in 100 μm PTFE (samples A-C);FIG. 38: 3 vol % 100 nm Ag in 100 μm PTFE (samples A-C);FIG. 39: 5 vol % 100 nm Ag in 100 μm PTFE (samples A, D);FIG. 40: 2 vol % 100 nm Ag in 1 μm PTFE (sample A);FIG. 41: 8 vol % 100 nm Ag in 1 μm PTFE (sample A); andFIG. 42: 10 vol % 100 nm Ag in 1 μm PTFE (samples B, C).

The experimental data is presented as a function of temperature forthree representative frequencies of approximately 10 Hz, 1 kHz and 0.1MHz, spanning the tested range. The data from 100 nm Ag/100 μm PTFEcomposites (FIGS. 37 to 39) demonstrate that the temperature dependenceof the conductivity varies markedly as the concentration of the 100 nmAg component is increased through the percolation transition. This isthe same, in general, for repeat tests. In some cases, at the lowestfrequencies, the real permittivity can become very noisy. This isusually for composites that are developing into conductive materials,such that the dielectric loss tangent diverges with decreasingfrequency, and exceeds the operational range of the measurementequipment.

A high conductivity that is inversely proportional to temperature, fortemperatures above the Debye temperature (215K for Ag), may berepresentative of the temperature dependence expected for a metal.

For 1 vol % 100 nm Ag/100 μm PTFE (FIG. 37), the conductivity andpermittivity is observed to be frequency dependent and to increase withtemperature above a particular temperature. This trend is most obviousat lower frequencies, and potentially marks the onset of the percolationtransition. The temperature dependence above this transition temperaturemay be due to “hopping” conduction mechanisms, discussed below.

For 3 vol % 100 nm Ag/100 μm PTFE (FIG. 38), the conductivity isobserved to be frequency independent, characteristic of being above thepercolation threshold. The conductivity initially decreases slowly withincreasing temperature, but then undergoes a further increase innegative gradient before rapidly increasing. As the data is presented ona logarithmic scale, these trends cover a small magnitude range.

For 5 vol % 100 nm Ag/100 μm PTFE (FIG. 39), the temperature dependenceof the conductivity is similarly complex. Both samples tested show twoturning points.

It was expected that samples near the percolation threshold couldundergo a rapid thermally induced insulator-metal transition during themeasurement, although this was not observed in the 1, 3 or 5 vol %samples. Therefore samples with 2 vol % 100 nm Ag in 100 μm PTFE weretested (FIG. 37).

It was observed that, in contrast to the other compositions tested, theproperties of individual samples for 2 vol % 100 nm Ag varieddramatically, and were not at all consistent. For example, sample A wassomewhat anomalous in that the conductivity was frequency independent,as if above the percolation threshold. Furthermore, the conductivityexhibited a maximum before rapidly decreasing at higher temperatures.This may be due to the percolation network being broken as thetemperature increases in the higher temperature range due to theexpansion of the matrix PTFE particles.

The conductivity of sample B exhibited comparable frequency andtemperature dependence to that of the 1 vol % 100 nm Ag samples, and soalso potentially provides evidence for hopping conductivity. However, amaximum conductivity is also found at an elevated temperature.

The conductivity of sample C exhibited several discontinuities,indicative of the sample undergoing repeated insulator/metal transitionsduring the measurements, although these inconsistencies were notobserved on the repeat tests.

A possible explanation for the changes in the direction of theconductivity gradient is that the percolating networks of silverparticles are disrupted or reinforced as the PTFE matrix particlesexpand. Negative gradients would be consistent with disruption of thenetwork, and positive gradients with reinforcement of the network.Conventionally, for particles dispersed in a continuum matrix, with aparticle size ratio, r_(filler)/r_(matrix)→∞, it would be expected thatthe network would be disrupted as the matrix phase expands. However, inthe opposite limit, r_(filler)/r_(matrix)→0, for excluded volumesystems, it may be possible for both behaviours to exist. For fillerparticles dispersed over the surface of a matrix particle, the fillerparticles may tend to be separated as the particle expands and thesurface area increases. However, this action might also tend to forcefiller particles distributed over the surface of one matrix particle tocome into greater contact with another matrix particle on the surface ofan adjoining matrix particle. This may reform the network or change thecontact resistance. For the more highly loaded composites, in which thematrix particles are densely covered, the latter effect may dominate.Such a reinforcing mechanism would be completely absent in the silvercoated microsphere paraffin wax composites, and should be less apparentin the 1 μm PTFE composites.

100 nm Ag/1 μm PTFE composites were also tested to enable a comparisonthat would reveal any differences that could potentially be associatedwith the difference in silver particle contact between the two systems.Representative experimental data is shown in FIGS. 17-19. Theexperimental data for the 1 μm PTFE composites is broadly consistentwith that for the 100 μm PTFE composites.

The data for the 2 vol % 100 nm Ag/1 μm PTFE (FIG. 40) is indicative ofbeing further below the percolation threshold than that for 2 vol % 100nm Ag/100 μm PTFE (FIG. 14) due to the absence of a temperature abovewhich the conductivity and permittivity are seen to increase.

The data for 8 vol % 100 nm Ag/1 μm PTFE composites (FIG. 41) is perhapsmore closely comparable to that for the 1 vol % 100 nm Ag/100 μm PTFEcomposites (FIG. 36) as a temperature above which the conductivity andpermittivity increases is obvious. The conductivity is also close to amaximum at the highest temperature tested, a feature that was observedin the 2 vol % 100 nm Ag/100 μm PTFE samples (FIG. 37). It is of someconcern that the turning points observed in the temperature dependenceclosely match the phase transition temperatures for water (freezing andboiling points), but no step discontinuities are observed. The sampleswere dry blended under an inert atmosphere before moulding and testing.

The data from 10 vol % 100 nm Ag/1 μm PTFE composites (FIG. 19) issimilar to the 5 vol % 100 nm Ag/100 μm PTFE composites (FIG. 16).Interestingly a broad peak is observed in the conductivity for sample Cfor this composition.

The various 100 nm Ag-based composites tested therefore show adifference in conductivity to the Ag-coated 15 μm spheres and paraffinwax composition tested in FIG. 2. The temperature dependence of theconductivity for the composites identified above as being driven by ahopping mechanism were analysed in the context of the Austin-MottActivated Polaron Hopping (APH) and Variable-Range-Hopping (VRH) models.The temperature dependence for each model is given by equations (31) and(32):

$\begin{matrix}{{\sigma_{a\; c}\left( {\omega,T} \right)} \propto {\omega^{s}{^{{({- \frac{W{({1 - s})}}{k_{b}T}})}\mspace{11mu}}\left( {A\; P\; H} \right)}}} & (31) \\{{\sigma_{a\; c}\left( {\omega,T} \right)} \propto {\omega^{s}T^{''}\mspace{11mu} \left( {V\; R\; H} \right)}} & (32)\end{matrix}$

The data from a selected portion of the temperature range, from FIGS.36, 37 and 41 is re-plotted in FIGS. 43-45, respectively, asln(conductivity) against the reciprocal of temperature, andln(temperature) to determine the activation energy W and the temperatureexponent n as defined in equations 31 and 32 (Menon R, Yoon C, Moses Dand Heeger A, Chapter 12 “Metal-insulator transition in doped conductingpolymers” in Handbook of Conducting Polymers, 2^(nd) Edition, Ed.Skotheim T et al, Marcel Dekker, New York 1998). The values obtained at10 Hz are summarised in Table 7.

TABLE 7 Composite −W(1 − s)/k_(B) n 1 vol % 100 nm Ag/100 μm −2200 6.9PTFE 2 vol % 100 nm Ag/100 μm −1283 4.4 PTFE 8 vol % 100 nm Ag/1 μm−3395 10.9 PTFE

The activation energy and temperature exponent appear to decrease withincreasing filler concentration, which is consistent with a decreasinginter-particle separation and hence a reduced barrier to hopping.Depending on the value of s (defined in equation 24 above), the valuesobtained are in reasonable agreement with values reported forintrinsically conducting polymers. The activation energy and temperatureexponent are large for composites comprising 1 μm PTFE particles,suggesting that the large particle size ratio in the 100 μm PFTEcomposites promotes tunneling, allowing the hopping conduction mechanismto occur more easily. Low frequency dispersion is also observed, whichcauses difficulties with the extraction of dc data to determined thedimensionality of the hopping mechanism.

The gradient of the percolation transition can therefore be altered bychoosing filler and matrix particles with a large size ratio. Byaltering the gradient, it is possible to reliably produce compositematerials that have a particular conductivity range. As the gradient ofthe transition is relatively flat, the conductivity will not beinfluenced, or influenced to a small extent, by compositional variationsresulting from the production process used to make the materials, forexample, weighing errors. The reliable temperature dependence of themeasured conductivity of the samples is also useful in situations wherenon-ambient temperatures need to be measured. Such tailored compositematerials are therefore of use in a wide variety of applications, suchas sensors for measuring temperature, pressure or concentration ofabsorbed chemicals. The external stimulus could also be electric fieldor current (which may cause heating).

For example, as the degree of connectivity between the electricallyconductive regions is increased when an external stimulus is applied tothe composite material. Such materials could then be used as sensors,actuators or switches, if the stimulus is applied dynamically.Alternatively in a passive form, the material could realise aconductivity that enables antistatic, electrostatic discharge,electromagnetic shielding products.

Although the materials discussed above have comprised silver orsilver-based conductive components, other suitable materials, could beused. For example, the electrically conductive material could be one ofmetal, metal alloy, conductive metal oxide, intrinsically conductivepolymer, ionic conductive material, conductive ceramic material or amixture including one or more of any of these. Alternatively, anoxidation resistant metal, a metallic alloy, a conducting ceramic or amixture including one or more of any of these could be used. Thenon-electrically conductive material could be PTFE, paraffin wax, athermosetting material, a thermoplastic material, a polymer, air, aninsulating ceramic material, glass or a mixture including one or more ofany of these.

The theories developed by Maxwell-Garnett and Bruggeman and discussedabove with reference to FIGS. 5 to 8 can generally be considered asconcerning the forming of three-dimensionally connected networks. Thecomposite production described above can also result in threedimensional materials. However it is only necessary for an incidentelectromagnetic wave to have an electric field component in a directionof connectivity for the effect to be observed. Hence, anisotropiccomposites with connectivity in two dimensions or even one dimensioncould suffice. Composites with two-dimensional or one-dimensionalconnectivity in the plane perpendicular to the plane of incidence wouldbe particularly useful. In this context, printing, etching andlithographic techniques, such as photolithography could be employed toproduce two-dimensional connectivity rather than the three dimensionalconnectivity which the methods described would produce. Printed layerscould then be laminated to form a bulk composite.

FIG. 46 is a schematic graph of conductivity in relation to conductivefiller concentration for a composite material comprising conductiveparticles in an insulating or non-conductive filler. The graphillustrates that the conductivity of the samples falls into 3 distinctregions, marked A, B and C.

In region A, the filler concentration level is low, and the materialdoes not conduct any electrical current. There are no connected pathwaysof conducting elements in the composite.

In region B, an insulator-conductor transition occurs. This transitionis prompted by the formation of the first network of conducting elementswithin the material. For dc use, this network must span the entirematerial. For ac use, the network need only span a region of thematerial. The steepness of the gradient in region B is determined by thedifference in conductivity between the constituent materials, theconcentration of the conducting elements at which the first networkforms and the concentration of the conductivity elements at which theoverall conductivity becomes limited by the contact resistance betweenadjacent conductive elements.

The gradient of the insulator/conductor transition (region B) can beinfluenced by the degree of randomness in the distribution of theconducting elements and the nature of electrical charge transport acrossthe contact interface. For example, the gradient can be influenced ifthe electrical charge transport is dominated by charge hopping ortunneling rather than essentially free-electron movement.

In the transition region B, the conductivity continues to increaserapidly as additional parallel paths of conducting elements are createdin the principal network through the successive addition of conductingelements. This is the percolation region.

Eventually, the gradient reduces to a plateau or saturation region C inwhich the further addition of conducting elements does not significantlyincrease the conductivity of the composite. In region C, the fillerconcentration is high enough for the composite to conduct electricity ata level similar to that at the conductivity elements. Typically, in thisregion the composite is useful as an electrical conductor.

A composite material is produced by printing or placing a pattern ofconductive elements onto an insulating film substrate. The conductingelements could be formed from any conductive material, including metals,conducting metal oxides, graphitic material, fullerenes, organicconductors or ionic conductors. The insulating film substrate could beformed from any insulating material including natural or syntheticpapers, cloth, fabrics or thin polymer films.

The pattern of conductive elements or particles may be printed or placedusing any pattern transfer mechanism or method whereby a thin layer ofthe conducting material can be placed in a controlled manner on asurface to form a user defined pattern. The possible methods involveinkjet printing, screen printing, block-foil patterning or autocatalyticdeposition such as described in WO 02/099162 and WO 02/099163, orphysical or chemical disposition methods. In the case of printingmethods, conducting particles would be dispersed in a low viscositybinder to enable deposition on the substrate. Alternatively, conductingmaterial could be removed from an initially complete conducting film toproduce a similar pattern of conducting material. The possible removingmethods include etching or hole punching.

The size of the conducting elements making up the pattern is ofsecondary importance and would be chosen to be smaller than the area ofthe substrate or area over which the composite is to be used, whicheveris the smaller. Typically, the element size would be less than one tenthof this size limit, and preferably less than one hundredth.

A pre-determined pattern representing a selected concentration ofconductive material is stored as part of a library of pre-determinedpatterns each representing selected concentrations of conductivematerials. These pre-determined patterns may be determined eitherempirically or theoretically. A combination of both theory andexperience in which a basic pattern is generated theoretically beforebeing empirically checked is a possible way of generating pre-determinedpatterns.

The pre-determined patterns are chosen or selected so as have particularproperties in particular circumstances. For example, the library ofpatterns may include patterns which when used to print or place an inkcomprising elements of a particular conductor (e.g. copper) of aparticular size and shape (e.g. discs of diameter 1.6 mm—see FIG. 2 a)on a particular substrate (e.g. synthetic paper) have a conductivityfalling within a particular small range ΔS (see FIG. 1).

There are likely even with the method of the present invention to bestatistical variations from one sample to the next but they will besignificantly smaller than the variations in the properties of thematerials made by the known mixing methods. In other words the standarddeviation of the conductivity of sample composite materials of aparticular conductor concentration produced by the method of thisapplication will be significantly smaller than the standard deviation ofthe same apparent composite material produced by the known methods. Thismeans that the behaviour of different samples will be closer andtherefore materials can be made with more confidence that propertieswill be repeatable.

FIGS. 47 a to 47 c illustrate a number of pre-determined patterns madeup of a 100×100 array including discs 1 of circular material,corresponding to, respectively, 20%, 50% and 70% loadings of conductiveelements.

FIG. 48 illustrates a pre-determined pattern made up of crossed dipoles2 and corresponding to a loading concentration of 50%. The aspect ratioof the crosses could be used, for example, to control the percolationthreshold of a composite.

FIGS. 49 and 50 illustrate the three stage autocatalytic depositionmethods described in WO 02/099162 and WO 02/099163 to which referenceshould be made. The contents of these two publications are hereinincorporated by way of reference and as illustrations of how thepreferred embodiments of invention might be implemented or created.

Turning to FIG. 49, an ink jet printing system 3 coats a substrate 4with an ink formulation containing a deposition promoting material in auser determined pattern 5. The treated substrate 4, 5 is then immersedin an autocatalytic deposition solution 6 to produce a user determinedmetalised pattern 7.

Ink jet printers operate using a range of solvents normally in theviscosity range 1 to 50 centipoise.

Turning to FIG. 50, a screen printing system 8 coats a substrate 4, withan ink formulation containing a deposition promoting material in a userdetermined pattern 5 (like numerals being used to denote like featuresbetween FIGS. 4 and 5). The treated substrate 4,5 is once again immersedin an autocatalytic deposition solution 6 to produce a user determinedmetalised pattern 7.

A range of ink formulations are possible. Criteria suitable for printingmay include the following:

-   -   1) They contain materials that are able to pass through the        chosen printing mechanism (for example, either an Epson 850        inkjet system or a Dek screen printer);    -   2) They contain liquids with the correct properties for the        printing process, for example suitable viscosity, boiling point,        vapour pressure and surface wetting;    -   3) Where suitable they contain binders and fillers affecting        either the viscosity or physical printing properties of the        printed ink.

The patterns of conductive material may also be transferred onto anon-conductive substrate using a straightforward printing technique suchas that described by Messrs Schwartz and Ludwena in “An experimentalmethod for studying two-dimensional percolation”. [Am. J. Phys 72(3),March 2004 © 2004 American Association of Physics Teachers] MessrsSchwartz and Ludwena describe an experimental technique for analysing arange of two-dimensional problems. The method is based on the printingof computer generated patterns using conducting ink. The metal-insulatortransition is measured from the print out of the conductive patterns,and the conductivity critical component and the percolation thresholdare calculated from these measurements.

Three-dimensional composite materials may be made by placing a secondlayer of insulating material over the material of FIG. 49 c or 59 c andthen repeating the printing process. The process may be repeated as manytimes as are necessary to achieve the desired material thickness orproperties. Such a material will, essentially, be three dimensional interms of its physical shape but as the insulating layers are continuousit will only be two-dimensional in so far as its electrical propertiesare concerned. Materials being three-dimensional insofar as theirelectrical properties are concerned may be created by connecting themetalised pattern of adjacent coated substrate layers 4, 5. Theconnection could be done using conductive vias through the insulatingmaterial separating adjacent metalised or conductive patterns.

The present invention allows for increased confidence in themanufacturing of composites having particular properties. This has anumber of clear advantages including the reduction of scrap.

Embodiments of the invention can, as discussed above, be used toengineer composites having, inter alia, desirable electrical, magnetic,thermal and/or physical properties. Possible applications of compositesincluding active materials (e.g. photo sensitive, piezoelectric,chemical sensitive, thermally sensitive) include sensors, actuators orswitches. Composites embodying the invention could also be used asreference materials (for e.g. absorbing) in metrology in support ofnational and/or international traceability claims. The ability toproduce something having a known and pre-determined property orbehaviour could also be used in support of security andanti-counterfeiting measures.

For example, WO02/099163 and WO02/009162 (both assigned to QinetiQLimited) disclose methods of autocatalytic coating and patterningrespectively. This is a form of electroless plating in which metals, forexample, cobalt, nickel, gold, silver or copper are deposited onto asubstrate via a chemical reduction process. Non-metallic surfaces may becoated following suitable sensitisation of the substrate. Pre-determinedareas of the substrate may be prepared for coating, allowing variouspatterns to be formed. Such patterns are printed onto the substrateusing pattern transfer mechanisms such as printing using autocatalyticinks. This would enable a number of random or non-periodic patterns tobe printed on single sheets, formed into a composite material bylaminating, and which would then exhibit a plasma frequency, similar tothose described below for 3-dimensional composite materials. Suitablesubstrate materials include insulating sheet materials, such as paper,card, polymer film or cloth.

The composite materials of the embodiments of the invention may be usedin various applications. One important use would be to combine thecomposite material with another material which has a magneticpermeability of less than 0, to produce a material with a refractiveindex of less than 0. Using the composite material to produce a materialwith a refractive index between 0 and 1 (less than air) would also be ofuse, since this would allow the formation of components exhibiting totalinternal reflection.

The composite material is also suitable for filtering applications,including those which require a tuneable filter. Such filter behaviourmay be coupled with various DC frequency applications. This may be usedto produce transparent or absorbing electrodes, capacitors or inductors.Transparent electrodes would be of particular use in microwave chemistryapplications.

The fact that composite materials of the type embodying the inventioncan demonstrate D.C. conductivity comparable with conventional metalswhilst remaining microwave transparent (behaving like a normaldielectric) is of potential usefulness. These potential usefulproperties can be engineered into materials using the processingdescribed. The advantageous behaviour arises from the percolatingnetworks of conducting particle being arranged in a suitable geometry.Consequently if this geometry can be altered by physical, thermal orelectrical deformation then these properties can be tuned or switched onand off depending on the desired application. Possible applications ofthe composite materials therefore include tunable high pass filters,commercial microwaveable food packaging, mechanically, thermally orelectrically switchable microwave filters for use in radomes or otherapplications requiring microwave spectrum selectively (e.g.telecommunications). Details of how to make products or devices foracting on or processing electro-magnetic waves are well known to thoseskilled in the art and easily found in relevant textbooks such as “TheElectrical Engineering Handbook”, (Editor-in-Chief, Richard C. Dorf;Publisher CRC Press Inc of Boca Raton, Fla.).

Examples of possible products which might use the composite materialinclude:

-   a) a written directional coupler lens—a negative permittivity in    concert with a negative permeability would lead to a negative    refractive index material. Such a ‘left handed’ material would    possess unique refraction properties allowing, for example, a flat    lens that would allow perfect image projection with no aberrations    due to geometrical shape as in a conventional lens. Such effects    are, of course, highly dispersive limiting the device to    monochromatic operation.-   b) filter—simple variation of the conductor/insulator morphology    within the composite can raise or lower the plasma frequency of the    material by several orders of magnitude. Therefore the cut-off    frequency where radiation can propagate through the medium (where    the permittivity crosses from negative to positive across the plasma    frequency) can be varied thus allowing easy fabrication of a    tuneable high pass filter device.-   c) transparent electrode—in electrically addressable devices such as    frequency agile sensors, the ability to apply an electric field    across such a device without any wavelength feature related    artefacts or attenuation occurring is very desirable. Thus, the high    conductivity conventional dielectric behaviour (positive    permittivity) above the plasma frequency allows the application of    ˜kHz driving electric field across a metal-like conductor whilst    allowing transmission of ˜GHz microwave radiation through a    conventional dielectric.-   d) absorbing electrode—as above, optimisation of the plasma    frequency allows fine control over the sign and magnitude of the    complex permittivity of the composite device to provide easily    customizable dielectric properties.-   e) capacitor or inductor—as above, straightforward    permittivity/impedance/admittance manipulation can realise such    devices.-   f) waveguide—the low permittivity behaviour frequency regime    behaviour of these composite materials above the plasma frequency    allows microwave propagation through a slab of such material with    total internal reflection occurring off the composite/air interface    exploiting the positive, sub-unity value of permittivity close to    but just above the plasma frequency. Such behaviour is highly    dispersive but this is not a problem in monochromatic    telecommunications frequency applications.-   g) sensor—the transition from insulating to conducting behaviour via    the percolating region of interest in this patent can be tuned to be    very sharp or a much gentler process. By careful choice of insulator    conductor concentration and processing conditions, a composite can    be achieved where the width of the percolating region is very    sensitive to electrical, mechanical or thermal perturbation. Thus,    relatively small changes in driving field, force or temperature can    induce relatively large changes in plasma frequency and related    dielectric properties. Hence, a high Q-factor sensor can be    fabricated.-   h) remote interrogation sensor package—as above, a switchable filter    device could be incorporated into a potential quantum cryptography    application.-   i) radome—typically, a radome needs to have durable physical    properties to house the microwave device within. In addition to    this, radar absorbing material (RAM) is included—often as a backing    applique. If the electrical properties (complex permittivity and    admittance) of the composite used in the structural part of the    radome could also be used in the RAM, then substantial weight and    complexity savings could be achieved.-   j) switch or shield—as above, tuning of the width of the insulator    to conductor transition could be exploited to make the device    sensitive to electrical, mechanical or thermal perturbations thus    realising a switchable device.-   k) fuse—as above, manipulation of the insulator/conductor transition    would enable a thermal or electrical (or mechanical) solid state    switch.-   l) anechoic chamber—as above, precise tuning of the electrical    properties (permittivity, admittance) of a material allows stringent    absorption and reflection design criteria to be met cheaply and    easily.

The composite material may also be used as a sensor, possibly as aremote interrogation sensor, where the plasma frequency is monitored byinterrogation by microwaves, in order to determine the state of thesensor.

As mentioned above, uses include materials for use in the food industry,for example, to aid heating or to provide packaging for microwaveablefoods.

Various other modifications are possible and will occur to those skilledin the art without departing from the scope of the invention which isdefined by the appended claims.

1. A composite material comprising a proportion of a electricallynon-conductive material and a proportion of a randomly distributedelectrically conductive material, and wherein: a) the electricallyconductive material is particulate with an average particle size whichis not more than 1 μm; b) the electrically non-conductive material isnot particulate; c) the electrically conductive material proportion issufficiently large to provide for the composite material to exhibit aplasma-like response corresponding to at least an onset of a percolationthreshold; and d) the composite material has a plasma frequency which isbelow the plasma frequencies of conventional bulk metals.
 2. A compositematerial according to claim 1 wherein the electrically conductivematerial comprises gold or silver particles with average particle sizein the range 1 μm to 1 nm.
 3. A composite material according to claim 1wherein the electrically conductive material has an average particlesize of 100 nm.
 4. A composite material according to claim 1 wherein theplasma frequency is not greater than 10¹² Hz.
 5. A method of influencingpropagation of electromagnetic radiation having a radiation frequency byarranging for the radiation to be incident upon a composite material,and wherein: a) the composite material comprises an electricallynon-conductive material and an electrically conductive material; b) theelectrically conductive material is randomly distributed in thecomposite material; c) the composite material exhibits a plasma-likeresponse and has a plasma frequency which is below conventional bulkmetals' plasma frequencies; and d) the plasma frequency is locatedrelative to the radiation frequency such that radiation propagation atthe radiation frequency in the composite material Is affected by theplasma-like response.
 6. A method according to claim 5 wherein theradiation and plasma frequencies are in at least one of the ranges 10³to 10¹⁵ Hz, 10⁸ to 10¹⁵ Hz and 10⁸ to 10¹² Hz.
 7. A method according toclaim 5 wherein the radiation and plasma frequencies are microwavefrequencies.
 8. A method according to claim 5 wherein: a) theelectrically non-conductive and conductive materials are bothparticulate; b) the electrically conductive material has a particle sizewhich is less than one tenth that of the electrically non-conductivematerial; c) the electrically non-conductive material has an averageparticle size which is not more than 100 μm; and d) the electricallyconductive material is a proportion of the composite material which issufficiently large for the composite material to exhibit a plasma-likeresponse indicating that it is at least at an onset of a percolationthreshold.
 9. A method according to claim 5 wherein: a) the electricallyconductive material is particulate; b) the electrically non-conductivematerial is not particulate; c) the electrically conductive material hasan average particle size which is not more than 1 μm; and d) theelectrically conductive material proportion is sufficiently large toprovide for the composite material to exhibit a plasma-like responseindicating that it is at least at an onset of a percolation threshold.10. A method according to claim 5 wherein the electrically conductivematerial has an average particle size of substantially 100 nm.
 11. Amethod according to claim 5 wherein the electrically conductive materialexhibits no long range order.
 12. A method according to claim 11 whereinthe electrically conductive material exhibits no long range order over aregion having a dimension in the range 3 mm to 3 m.
 13. A methodaccording to claim 12 wherein the region's dimension is substantially 3cm.
 14. A method according to claim 5 wherein the electricallyconductive material exhibits no long range order over a region having adimension of the order of a wavelength in the material corresponding tothe plasma frequency.
 15. A method according to claim 5 wherein thecomposite material contains sufficient electrically conductive materialto form therein at least one electrically conductive network extendingbetween opposite sides of the composite material.
 16. A method accordingto claim 5 wherein the electrically non-conductive and conductivematerials are both particulate and have larger and smaller averageparticle sizes respectively relative to one another, and a ratio of theaverage particle sizes being greater than or equal to 100 or
 1000. 17. Amethod according to claim 16 wherein the electrically conductivematerial comprises one of an oxidation resistant metal, a metallicalloy, an electrically conductive coating on electrically non-conductiveparticles and a conducting ceramic, and the average particle size of theelectrically conducting material is in the range 1 nm to 1 μm.
 18. Amethod according to claim 17 wherein the electrically conductivematerial has a conductivity greater than 1 S/m.
 19. A method accordingto claim 17 wherein the electrically conductive material comprises goldor silver particles.
 20. A method according to claim 5 wherein theelectrically non-conductive material comprises either one of, oralternatively a mixture of at least two of, PTFE, paraffin wax, athermosetting material, a thermoplastic material, a polymer, air, aninsulating ceramic material and glass.
 21. A method according to claim20 wherein the electrically non-conductive material is PTFE with anaverage particle size of substantially 100 μm, and the electricallyconductive material is gold or silver with an average particle size ofsubstantially 100 nm.
 22. A method according to claim 5 wherein theelectrically conductive material comprises either one of, oralternatively a mixture of at least two of, a metal, metal alloy, anoxidation resistant metal, a conductive coating on non-conductiveparticles, an electrically conductive metal oxide, an intrinsicallyconductive polymer, an ionic conductive material, and a conductingceramic.
 23. A method according to claim 5 wherein the compositematerial has an effective conductivity exceeding at least one of 10 S/m,30 S/m and 100 S/m.
 24. A method according to claim 5 includingswitching the composite material between radiation propagating andattenuating states by altering its plasma frequency.
 25. A methodaccording to claim 5 wherein the electrically conductive materialcomprises regions of electrically conductive particles and theelectrically non-conductive material comprises regions of electricallynon-conductive particles, and the composite material has a degree ofelectrical connectivity between the regions of electrically conductiveparticles determining electrical properties, and the method includesapplying a stimulus to the composite material to change the degree ofconnectivity.
 26. A method according to claim 25 wherein the stimulus ispressure, temperature, chemical absorption, electric field or electriccurrent, and applying the stimulus switches the composite materialbetween radiation propagating and attenuating states.
 27. A methodaccording to claim 5 wherein the composite material is in the form ofany one of a directional coupler lens, filter, transparent electrode,absorbing electrode, capacitor, inductor, waveguide, sensor, remoteinterrogation sensor package, active electromagnetic shutter, radome,switch, shield, fuse and anechoic chamber.
 28. A method according toclaim 5 wherein the composite material is one of a series of compositematerials with differing concentrations of electrically conductive andnon-conductive materials, and wherein for the series a graph ofconductivity against electrically conductive material concentration onlogarithmic axes has a slope which is less than 100 for an insulator tometal transition.